One day you just stop getting comps. No explanation. You can go to a host and tell them you had a fight with one of the pit bosses over some nonsense, the guy was being an a-hole, and you think he may have put something on your account. Sometimes they'll tell you what's on there. Pick a young looking new host.
You cannot force a casino to give you comps (or honor coupons). They are always at the discretion of the casinos which is why many casinos won't tell you your comp balance. Comps and Coupons are always subject to being canceled. Cashback on the other hand has been deemed to be a contractual thing...you play X action and get Y cashback. Therefore most casino jurisdictions don't allow casinos to take your accrued cashback.
Harrah's Reward Points are both comps and cashback. (now that they can be cashed out at 50c on the dollar). I think that there would be a problem in most jurisdictions with Harrah's if they tried to take them and the player fought it. Harrah's of course does zero out your discretionary comps and will often revoke your Diamond or 7-Star status leaving you at Gold and no-mail status.
Foxwoods and others are governed by their own laws (tribal law) and if they decide to take your points there is probably little you can do about it.
The Moral of the Story...keep your cashback and comp balance low by using them up as you earn them and only keeping a small balance for the occasional meal or incidental charge. Big outstanding comp balances make it even more likely that you will get backed off because the casino has a greater incentive staring them in the face.