Cannot do it today. This has long been on the list of things we want to implement, but haven't been able to have the time to do it (there is alot involved to being able to do this):
For now, you have to do some manual things, I *think* this is everything:
1) shutdown your agents
2) uninventory the platforms that have resources whose types came from the plugin you want to remove
3) shutdown your servers
4) remove the plugin .jar file all your servers' file systems (jbossas/server/default/deploy/rhq.ear/rhq-downloads/rhq-plugins)
5) Using a DB client tool (pgAdminIII, dbVisualizer, sqlplus, whatever), you need to remove the plugin row from the database table RHQ_PLUGIN ("delete from RHQ_PLUGIN where name = 'your plugin name as defined within its descriptor')
6) Using same DB tool, remove all resource types that were defined by that plugin - "delete from RHQ_RESOURCE_TYPE where plugin = 'your plugin name"
7) Restart your servers
8) Restart your agents with the --cleanconfig option
I think that's it.