1. Deinstall rac database :
cd $ORACLE_HOME/deinstall ./deinstall
2. Verify the status of the cluster and observe that the database is not available as resource in clusterware.
crsctl status res -t
3. Now login to grid ORACLE_HOME and deinstall rac clusterware.
. oraenv +ASM1 cd $ORACLE_HOME/deinstall/ ./deinstall
https://hiteshgondalia.wordpress.com/2012/11/26/how-to-manually-drop-database-in-oracle-rac-environment/
http://www.askmlabs.com/2014/03/uninstallingdeinstalling-rac-software.html