The Database host has refused your connection. Verify that the connection proprieties (HOST/PORT USER/PASSWORD etc.) are correct, and that the connection is not being blocked by a firewall.

an alternate form of this message is:

The host has refused your TCP/IP connection. Verify that the connection proprieties are correct, and that the connection is not being blocked by a firewall.

These are the known problems and solutions for this issue.  Many of these are addressed in the video embedded at the bottom of the article:

1) The database instance or listener are not running.
If the database is not started or isn't listening on this port on that host, a connection refused error will display.  Verify with the DBA that the database instance and listener are running and the service is registered with the listener.

2) TCP connectivity
a) Ensure that there is TCP connectivity from the machine where Control-M is being installed.  Once the hostname (or IP) and port are confirmed by the DBA, this can be tested executing the following command on the host where Control-M is being installed.
   telnet {db host} {listener port}
If this fails to connect or times out, you will need to contact your network administrator.
b) If the incorrect IP address for the database listener is being resolved with the hostname provided from the client, this will produce the same message.

3) The wrong DBA password was used.
Ensure that the DBA (system) password entered in the Database Properties panel is valid.  Contact your local DBA for assistance in verifying the password, as appropriate.

4) Local filesystem is not being used for Control-M 7 or 8.

This error may show up if the installation is being done on a NFS file system. Control-M version 7 and 8 installations are not supported on Network File Systems and need to be done on a local file system. After installing it on a local file system, some directories can be moved to NFS. Please see the Control-M Install Guide for these details. (KA344504)

5) Wrong service name or SID was used

When performing the installation to an existing Oracle database, if the user specifies an invalid service name this message is issued by the installer.  If you have ensured that there is no TCP connectivity issue, please confirm that the Oracle service name/SID specified during the installation is correct. You can do this my looking at the 'listener.ora' or running the 'lsnrctl status' command on the Oracle server.  Please have your DBA assist you with this.  The defect CAR00046204 has been opened to create a clearer message

6) When installing Control-M 7, using Oracle RAC a patch for the installation files is required.  This can be found @ ftp://ftp.bmc.com/pub/control-m/opensystem/CONTROL-M_Server_for_UNIX_and_Microsoft_Windows/7.0.00/PAOST.7.0.00.002/.

7) Oracle listener security may be preventing connection.  To discover if security is enabled, run the command 'lsnrctl status' and look for a line:
  Security ON: Local OS Authentication
Please refer to http://www.oracle-base.com/articles/misc/OsAuthentication.php for more details on OS security.  It may be necessary to define the Control-M user on the database server host if this option is not disabled.

8) The installation hostname is longer than 25 characters.  The defect CAR00043324 exists in the 7.0.00 installation and is corrected in 8.0.00 and later.

9) The Database Owner (DBO) user, profile, or role already exists.  Please ensure the username selected at the install time does not exist or the profile or role associated with this user if previously created.  Typically, the role and profile are named BMC_ROLE_{username} and BMC_PROFILE_{username}.  Your DBA should be able to verify their existence.

10) Control-M/Enterprise Manager version 8.0.00 and 9.0.00 now support Oracle Database 12c.  In order to enable this support, do the following: 
a. Before Control-M installation, open Oracle Database Server's sqlnet.ora (ORACLE_HOME/network/admin/sqlnet.ora) and verify that the following line is present: 
SQLNET.ALLOWED_LOGON_VERSION=8 
b. If the value is other than 8, change it to 8. This change is usually performed by the local site DBA. 
After the Control-M installation is complete, the change above can be reverted.

Note:
SQLNET.ALLOWED_LOGON_VERSION=8 is an old parameter suggest to DBA to use
SQLNET.ALLOWED_LOGON_VERSION_CLIENT=8

11) When installing against Oracle 12C Database instance with Oracle Security Banner enabled.

Following CAR has been opened to address this problem:

CAR00074568 - JDBC THIN DOES NOT CONNECT WHEN SECURITY BANNER IS SET IN DATABASE
At this stage we do not have any further details about planned release/date when this is going to be implemented.

As workaround temporarily disable Oracle Security Banner while installing the product.

12) When installing, the oracle system password contains a special character such as: 
  # ! * $
The password will have to be temporarily changed to a simple password (not containing special characters) to complete the installation.  This is defect CAR00036830.

13) Against Linux operating system only , the JDBC oracle connection can be prevented by the Linux operating system low entropy:

https://www.usn-it.de/2009/02/20/oracle-11g-jdbc-driver-hangs-blocked-by-devrandom-entropy-pool-empty/

The ouput of the following command should be above 1000 :
# cat /proc/sys/kernel/random/entropy_avail

if not , please  install and start rngd daemon as described in this article:

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-encryption-using_the_random_number_generator

After the Control-M installation completes successfully you can remove the daemon afterwards.

14) Check the sql client connectivity with the instructions of that knowledge article :

https://bmcsites.force.com/casemgmt/sc_KnowledgeArticle?sfdcid=000214655

15) Control-M is using an older version of the OJDBC client that the Oracle server is rejecting. 

Check the instructions of this knowledge article :

https://bmcsites.force.com/casemgmt/sc_KnowledgeArticle?sfdcid=000418969 

If none of the above resolves the issue, please send the following information to BMC Support:
1) output from uname -a
2) output from check_req.sh
3) output from the oracle check req script (check_req_oracle_db.sql)
4) the version of Oracle for the target database
5) the version of Existing Oracle Client for the EM user (if applicable)
6) the /etc/hosts file
7) the installation log file
8) is Oracle Server using ASM or RAC
9) Linux only : cat /proc/sys/kernel/random/entropy_avail