Please realize that the tone of this post is not meant to criticize you personally, but it is firm to get the point across.
You've let this go on for too long. This situation is not 100% the tenant's fault. Land lords who allow tenants to get behind month after month have--in effect--trained their tenants regarding what is acceptable behavior. So it is not surprising now that they pay everyone else FIRST and you LAST. You can bet their cable TV bill is paid. Why? Because the TV company cuts off the cable if not paid...but you've let them continue to make rent the bottom priority when it should be at the top along with food, utilities, and transportation. Realize that since you are part of the problem, only your firm, decisive action will be the solution.
They have already in effect "broken the lease." The question now is how much of a financial hit are you going to take. Here's what I'd do to minimize your losses:
Tell them they can move out in exchange for signing a Promissory Note for the December rent that is due. You can work out payment arrangements: divide it up over the next 6-12 months. They also must be out in the next 10 days, no "ifs", "ands" or "buts" so that you have time to clean and repair in time to hit the ground running January 1st. Do not discuss the security deposit except to say that proper accounting will be done, and any damages and money owed will be charged against the deposit. Therefore, if they leave the unit clean and undamaged, most if not all of the deposit can be applied towards the Promissory Note balance due, which will in turn reduce their payments over the next 6-12 months. IF they do ALL of those things, you will not sue them and they can get out of the lease.
Give them no more than 24 hours to think over and accept your offer. This includes signing that Promissory Note. That will be your ammunition in court if they fail to make payments. No arguments about rentals and leases...flat out this note says your promised to pay and you didn't...end of story.
If they refuse to sign the Note, get mean-spirited and argumentative, don't respond to your communications, demand the deposit back, trash the place...I would file eviction tomorrow. Seriously, I'd fax their lease to my attorney and say, "Get 'em out ASAP."
Good luck!
--108.250.xxx.xxx