Legal Question in Real Estate Law in California

You can cut them yourself.

Tree limbs or roots that go beyond the owner's property line are trespasses. Unlike most other forms of trespass, the law allows the victim of "tree trespass" to resort to self-help, without going to court, by trimming the roots or branches himself, at his expense. The self-help must be done in a "non-negligent manner," according to one authority. This probably means the trimming must be done in a way that the tree is not seriously injured, killed, or made liable to fall over in a storm.

So, your neighbor is probably acting within his rights.