1) Have your MIL return all letters to the sender.
2) With your cell phone provider, block the number they are calling from. If it's a private or unavailable number, do not answer.
3) Send them a letter telling them to cease and desist calling you. This letter needs to be sent CMRR, by itself! On the green card, before you mail it, write on there C&D letter so you will know what it refers to when the green card comes back.
4) Send them the dispute letter that RobertEG suggested, CMRR, by itself!! Do NOT include it with the other letter. On the green card, before you mail it, write on there Dispute so you will know what it refers to when the green card comes back.
Something I always do, is write this note at the bottom of all letters...
Sent Certified Mail, Return Receipt, #xxxxxxx <---- put in the actual number for the CMRR.
It lets them know I am tracking this, I am serious. It also helps match up the green card to the copy of the letter.