How to uninstall/remove EM12c Agent

./emctl stop agent

/opt/app/oracle/agent12c/core/12.1.0.3.0/perl/bin/perl /opt/app/oracle/agent12c/core/12.1.0.3.0/sysman/install/AgentDeinstall.pl -agentHome /opt/app/oracle/agent12c

rm -Rf /opt/app/oracle/agent12c

From host where emcli is installed:
emcli delete_target -name="test.example.com:1836" -type="oracle_emd" -delete_monitored_targets –async

Reference:
https://docs.oracle.com/cd/E24628_01/install.121/e24089/deinstall_agent.htm#CBBCEJHG

Check any left out info:
select target_name,target_type from MGMT_TARGETS_DELETE where DELETE_COMPLETE_TIME is null

select target_name,target_type from MGMT_TARGETS_DELETE where target_name like '%spare%' and target_type='oracle_emd'


You can try:
exec mgmt_admin.delete_target('target_name','target_type');


If the agent install fails to register with OEM than try:

     cd /opt/app/oracle/agent12c/agent_inst/bin/
    /opt/app/oracle/agent12c/agent_inst/bin/emctl secure agent
    /opt/app/oracle/agent12c/agent_inst/bin/emctl start agent
    /opt/app/oracle/agent12c/agent_inst/bin/emctl config agent addInternalTargets

   /opt/app/oracle/agent12c/agent_inst/bin/emctl upload