Following is the only step by step approach to drop EM repos.

Before that:

1. Ensure environment variable properly set.
2. Ensure your listener properly configured and started.
3. Ensure password file is created. (Optional)
4. You need SYS,SYSMAN & DBSNMP password.
5. Ensure your /etc/hosts file mentioned below two lines.

[oracle@prod ~]$ cat /etc/hosts
127.0.0.1 localhost.localdomain localhost
192.168.17.146 prod.localdomain prod

Environment Variable:

export ORACLE_SID=orcl
export ORACLE_UNQNAME=orcl
export ORACLE_HOME=/home/oracle/app/oracle/product/11.2.0/dbhome_1/
export PATH=$PATH:/home/oracle/app/oracle/product/11.2.0/dbhome_1/bin/

Instantiate following command to deconfig Database Control with the help of EMCA.

[oracle@prod bin]$ ./emca -deconfig dbcontrol db -repos drop
STARTED EMCA at Apr 4, 2015 3:48:11 AM
EM Configuration Assistant, Version 11.2.0.0.2 Production
Copyright (c) 2003, 2005, Oracle. All rights reserved.
Enter the following information:
Database SID: orcl
Listener port number: 1521
Password for SYS user: 
Password for SYSMAN user: 
Password for SYSMAN user: 
Do you wish to continue? [yes(Y)/no(N)]: Y
Apr 4, 2015 3:48:25 AM oracle.sysman.emcp.EMConfig perform
INFO: This operation is being logged at /home/oracle/app/oracle/cfgtoollogs/emca/orcl/emca_2015_04_04_03_48_10.log.
Apr 4, 2015 3:48:25 AM oracle.sysman.emcp.util.DBControlUtil stopOMS
INFO: Stopping Database Control (this may take a while) ...
Apr 4, 2015 3:48:31 AM oracle.sysman.emcp.EMReposConfig invoke
INFO: Dropping the EM repository (this may take a while) ...
Apr 4, 2015 3:50:13 AM oracle.sysman.emcp.EMReposConfig invoke
INFO: Repository successfully dropped
Enterprise Manager configuration completed successfully
FINISHED EMCA at Apr 4, 2015 3:50:17 AM

Cheers!! Database Control de-configure successfully.

Stay Tune. 🙂