If a tenant has not paid rent, you must serve him/her with a 14-day notice to quit. The notice must contain certain language and is best prepared by an attorney. The notice must also be served by certified or registered mail or by a constable or sheriff (preferably the latter two). If the tenant does not pay within the 14-day period, you may bring an eviction complaint. There is a timetable for doing that can best be explained by an attorney. Under no circumstances can you evict a tenant without what is known as an execution for possession issued by a court (and even then not until a sheriff or constable has provided the tenant with 48 hours advance notice that they will be evicted).
If your tenant is delinquent in paying his or her rent, Massachusetts law permits you to serve upon your tenant a 14-Day Notice to Quit. If the tenant has not been served a 14-Day Notice to Quit in the previous year, the tenant may cure the non-payment by making full payment of the rent to you within 10 days of receiving the notice to quit.
If the tenant does not cure within 10 days and after the 14 days has elapsed, you may start a summary process (eviction) action in district court or, if the property is in Acushnet, the Southeastern Housing Court.
You cannot simply throw out a tenant who does not pay rent (or change the locks or take any other self-help measures), as Massachusetts requires a full judicial procedure. Once you have won your case in district or housing court and have received an Execution (the piece of paper allowing you to move out your tenant) you must contact the county sheriff or a constable skilled in service of process to begin the process of moving your tenants out.
Be careful about evicting tenants who claim they are withholding rent because of unsanitary conditions. You may find yourself in the uncomfortable position of defending counterclaims for the breach of warranty of habitability, unlawful retaliation, etc. If the judge finds for the tenant, you may owe the tenant more than the tenant owes you....
If you need legal assistance in a thorny eviction case. consider retaining an attorney with experience in housing law.