Have the surveyor mark the property line, and tell them to remove it.
Putting a "fence" or improving abutting property is the first step to legally taking it for themselves. I think the correct term is by "eminent domain" but that may just be when a govt. body takes land.
In Massachusetts, if you have fenced in and "improved" (for at least 10 years) a piece of property abutting yours, you may make a legal claim that it is yours.
Also, if the surveyor lets the bank know, they may not give you a mortgage.
Have the septic tank checked and pumped out, you should always move into a house with a clean tank, so you know when it was done last. If there is any damage, get the neighbors to pay.