I'm currently a Real Estate agent, but I have also been a landlord for the better part of the last two decades. I have been a property manager as well.
The absolute FIRST thing I would do is have a "14-day Notice To Quit" served. It'll cost you a few bucks, but most people, when they start to realize the consequences of cheating their landlords, will see the light and pay up. If they don't or can't pay up, you have begun the eviction process and after 14 days elapse without payment you can proceed to the next step. The clerk at the courthouse can walk you through the process. Be certain to follow every step to the letter! The reason evictions can be dragged out for months and months is that the process isn't done properly/completely so you keep having to start over. I found a good overview for your consideration at http://www.rocketlawyer.com/document/massachusetts-eviction-
In the end, if you use the courts (which you pay for every time you write a check for property tax anyway, so why not use them?) you should receive a judgement in your favor. That judgement is something you can use to guarantee they actually move out some day, and is something you can bring to a credit reporting agency. The credit score factor will leverage your odds of actually seeing the money you are owed some day. Without you clearing their debt, finding a new place to live is not going to be a picnic for them, nor will buying a car, getting a credit card, cell-phone, etc... An eviction order will also show up as a civil court appearance and make future landlords unwilling to take a risk on them.
Hopefully this gets you going in the right direction. If you don't already have a working relationship with somebody in the Real Estate business, form one ASAP. If you throw some business their way every once in a while, they'll be there for you when you need some help with whatever issues come up.
Best Of Luck.