I suppose you are allied with everyone one else too.
If that is the case, follow these steps:
1) Disband most of your armies. This will save you huge amounts of money each turn.
2) Train dozens, if not more assassins, and quite a few spies.
3) Concentrate them on 1 factions. Attack captains first. When your assassins have better subterfuge due to the dead captains, assassinate all generals.
4) By now, there are few generals remaining. You probably couldn't kill the faction heir & faction leader. Since these are only 2 men, destroy the army in which they are present.
5) With all the family members gone, the factions turns into rebel settlements.
6) Capture the rebel settlements one by one. Since these don't belong to any faction, you won't provoke war with other factions.
Damn. All small error I noticed: a failed assassination will cause hostilities..
lame