What does your lease say about the condition of the premises?
What is "filthy"? I know some land lords who are meticulous housekeepers for whom one smudge on a wall or dishes left in the sink is "filthy."
The bottom line is dirt is a highly variable condition. Would a judge agree that "filthy" by Lloyd's definition = property damaging filthy? Many of my residents keep their units in a state of clutter, disarray, but some of them keep their units better than my personal residence. "Filthy" is in the eye of the beholder MOST times. However...
If municipal ordinances are being violated and/or you have a bug infestation, then perhaps you have an evictable lease violation. Keep in mind if you go to court you must PROVE it warrants someone being tossed out of their home. Unpaid rent is a lot easier to prove than "this is too dirty."
Just a few things to check...
Do they have established trash service?
Is there junk outside the unit cluttering up the lawn?
Are they storing kids toys or other "attractive nuissances" outside (pools, trampolines, playgrounds, etc)?
Do they have non-outdoors, upholstered furniture on the front porch?
I'd start with issuing a notice to clean up. If that fails, non-renew the lease.
--173.19.xx.xxx