Property line disputes can be the worst. I remember the was some psycho lady in the neighborhood where I was growing up and she had this property line grudge against one of the neighbors and made their life hell. At some point they just gave up and let her have the 5 extra feet she was claiming. It wasn’t worth the property damage and police visits.
Invest in a home security system with cameras installed facing this property line and try to work out some kind of deal with these neighbors. Of course with what they’ve done so far it doesn’t look good for you.