Well...........yes and no. If I were you I don't think I would stop payment on any checks or drafts you have already sent because that may give them a reason to sue you (on the stop payment). So you may want to consider that one gone (hopefully a cheap lesson).
Is NCO Group the ones collecting your debt?
The fact is - you are going to have this problem for a long time. Even if the company collecting right now stops, they will probably just sell the debt to another company who will try the same stuff. Keep copies of any letters on your computer so you can change the names when you are contacted by another company in the future, and become familiar with the FTC online complaint system, so once you do all this the first time you can do it again much faster when the debt is sold again.