How to break enemy alliances

[V.T. Marvin]

(...) Then, since the the computer will never break protectorate...

My experience show that the AI in fact does attack its own protectorates sometimes.
In my current campaign (as Pahlavans) Epeiros tried to delay its inevitable doom by offering themselves as Roman protectorate. The Senate always accepted but attacked next turn. This cycle repeated for several turns, until the powermongering Romans swallowed them completely.
Also in my first campaign (0.81) I tried to become a Seleucid protectorate to see what it does - only to be attacked by them anyway...