Neighbors encroaching on my land

[AJ Shepard]

I am having this issue at the first house that I ever bought.  I used to live in it and have since rented it out to college students.  It's on a 5k lot and previously had a fence separating my lot from my neighbor's lot.  A while back the neighbor asked me if they could replace the chain link fence with something like the white picket one in the front and I agreed (verbal).  Since then they have taken down a ton of my landscaping, built rock walls and planters on my property.  I'm not one to complain about a little thing, this is a massive amount of work they have had done.  Probably in the 5-10k worth of improvements, but all on my land.  They've covered up where my sprinklers once would come up. 

I've sent them a letter informing them to remove the items as my current set of renters has turned over and any agreement that they may have had with them was not allowed or approved by me.  I have not heard back from them, but I don't anticipate them doing anything other than continue to encroach on my land.  I'm looking for any suggestions on how to handle this other than go get an attorney and spend a bunch of money.  

Original Chain Link Fence:

Now: