Steps to Remove/De-register Agent from OEM:

1. Stop the Agent on the target machine:

cd /bin
emctl stop agent

2. Login to the Repository Ddatabase (via sqlplus) as user SYSMAN and execute the following command:
SQL> exec mgmt_admin.cleanup_agent(':');

For example:

SQL> exec mgmt_admin.cleanup_agent('ebsqa-mt.oracle_mt.com:3872');

PL/SQL procedure successfully completed.



3. De-install the Agent on the target machine.


Reference: Metalink Note - 438158.1 -- How to Cleanly De-Install the EM 10g Agent on Windows and Unix