I suppose some OOC agreement might be useful for banishing/exiling.
Although as Zim correctly said, this could defeat the purpose of it being an ability.
But reading Ramses' comment, I think adding following may make it fairer (and ensure that players do agree):
- Requires that the noble banished is currently involved in a civil war

This is because the FL can then say "OK, you guys are fighting within my country, and I think I favor X over Y, so Y is hereby declared an outlaw". If you are already in a civil war, its basically assumed that you agree to your character being involved in some more dramatic event.