My question involves real estate located in the State of: California

I live in Los Angeles. I bought my house 9 years ago. About 7 years ago my neighbor and I went halves on a brick wall. About 2 years ago my old neighbor moved out and someone esle moved in. My wife and I recently did a full remodel of our house. We had to get our property surveyed and found out the wall to the west of us is a half a foot into our property. The entire wall is on out propery. I told the neighbor about the survey. He has countinued to drill holes and plant vines on the wall. Today I saw a man drilling more holes to give support for climbing vines. What should I do? Should I write a legal letter ? If I take the wall down am I responsible for his plants?
I dont want to give up my property