Can MA landlord sue for damages after returning security deposit?

I recently moved out of a house I rented for 2 years. The landlord & his wife came the day I moved out, saying they'd walk through and this would speed up the return of my security deposit. They appeared happy & repeatedly said how great a tenant I'd been. About 3 weeks later, I got an angry call from the landlord's wife, full of vague implications that I might not get the full deposit back. I didn't hear anything that was valid or that I hadn't already OK'd with her husband, and told her so. She really seemed to be searching for reasons to keep the money. I then received the security deposit check from them, just before the 30 day limit. The only thing subtracted was the amount of the last water bill, as agreed, as that arrived after I'd moved out. I thought our business was complete, and deposited the check. Since then I've received another nasty phone call, and then a letter (the call had been so abusive I'd sent them a letter asking them not to call me again), saying they are going to bill me for all sorts of things. My question is: Can they actually take any action against me, or bill me, or continue to harrass me, after having returned the deposit?