Having owned and found boarding for two draft crosses I haven't had an issue finding a facility. The place we are at now was under the impression our horses ate MORE than your average horse... However, they in fact act LESS. She didn't have to grain or hay them during the summer and they were fat enough to be forced to muzzle them one day on and one off. They were on 3 acres of nice grass though.
So if pasture is good in the area, they should be able to debunk people hesitating to take on drafts.
Make on the subject though, I would follow what others have advised. Written notice that they have X amount of days to leave your facility. You've been more than generous with them and it is clear they don't appreciate what has been granted to them for a small favor in return. In the meantime, if you are willing to take new boarders, start listing again.
The Fluffy Equestrian
Rhett | Hang on "Fiona" | Rockstar
Reg. TWH Gelding | Irish Draught Horse Mare | SSH Gelding