Harrassment through Litigation in California

California has a vexatious litigant statue, CCP Section 391. It includes relitigiating the same issue over and over after it has been decided adversely. You might see if his actions fit that definition

391. As used in this title, the following terms have the following
meanings:
(a) "Litigation" means any civil action or proceeding, commenced,
maintained or pending in any state or federal court.
(b) "Vexatious litigant" means a person who does any of the
following:
(1) In the immediately preceding seven-year period has commenced,
prosecuted, or maintained in propria persona at least five
litigations other than in a small claims court that have been (i)
finally determined adversely to the person or (ii) unjustifiably
permitted to remain pending at least two years without having been
brought to trial or hearing.
(2) After a litigation has been finally determined against the
person, repeatedly relitigates or attempts to relitigate, in propria
persona, either (i) the validity of the determination against the
same defendant or defendants as to whom the litigation was finally
determined or (ii) the cause of action, claim, controversy, or any of
the issues of fact or law, determined or concluded by the final
determination against the same defendant or defendants as to whom the
litigation was finally determined.
(3) In any litigation while acting in propria persona, repeatedly
files unmeritorious motions, pleadings, or other papers, conducts
unnecessary discovery, or engages in other tactics that are frivolous
or solely intended to cause unnecessary delay.


The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.

I concur with Mr. Fink's suggestion about vexatious litigant rule. Also, if you go to verdict in any case in which he has sued you and you win you can sue him for malicious prosecution if the facts are such that he had no reasonable/probable cause to bring the action in the first place and did so maliciously and/or in reckless disregard of the fact he had no valid claim against you. His attorney can also be sued for malicious prosecution for signing the pleadings if you won the underlying case. So...you need to speak with an attorney about these options. You may also seek a restraining order and to have the visitation provisions changed in your divorce/custody matter.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive legal consultation and review of all the facts and documents at issue. This answer does not constitute an attorney-client relationship.

You need to bring this information before the court, and file a 128.7 motion, or a 391 motion. You can go to law library and ask a librarian to help you find forms, or you can hire an attorney (which is much better).