Legal Question in Real Estate Law in California

Send a certified, return receipt letter (keep a copy) to your neighbors, giving them a deadline for removing the encroaching satellite dish. If they don't do so, contact an attorney to go to court to make them remove it and compensate you for using your property without your permission (i.e. trespassing). Obtain a copy of your property survey to make sure you're correct in where your property actually ends.

While I generally agree with Mr. Cohen's answer, I'd prefer to deal with the neighbor in a little more gingerly way, if there's any hope it'll work. You may not be on that firm ground in a suit for trespass (if negotiations didn't work) because there is a statute of limitations for permanent trespasses (as opposed to so-called "continuing trespasses"), which this appears to be, so the neighbor may have a solid defense to any legal suit.