When you say you tried to remove the database, what steps did you follow and which database? The instance or the cluster database? What is your end goal?
Typically, you would remove the instances, then the cluster db. You wouldn't remove the host unless you're completely removing the system from EM all together, in that case the easiest way to remove the host and all targets is to use emcli to delete the agent and monitored targets (agent + host + db, etc.).
If the target is still pending deletion, you can do two things.
1. download emdiag repvfy kit and run repvfy verify targets -fix -level 9 (EMDIAG Omsvfy 12c Kit - Download and Install (Doc ID 1374450.1))
2. go to setup /manage cloud control/overview, find the link that says pending delete targets and then see if the target is still listed as pending, and try the force
If your cluster still exists, minus this host, go to the cluster target and go to monitoring configuration to ensure that the cluster only consists of the valid hosts afterwards.