Legal Question in Personal Injury in California

That does not rise to the level of civil harassment. You can just throw the letters out. It is a USPS regulation violation; good luck getting a postal inspector to care, though. The more relevant issue is whether the "neighbor" has a point. Failure to maintain property can be cited by the city for "blight." It can also give rise to a cause of action for private and public nuisance that they could sue you for if you are, actually, allowing unabated weed growth on your property. But if you are maintaining your property, and all the neighbor is doing is putting letters in your mailbox, just throw them out and move on.