So are you working at the moment for free? They could up and leave right now because they haven't paid? These people are more trouble than they are worth.
What kind of notice period do you have in your contract?
I don't think they have given you enough of a reason to terminate without notice. I am getting the sense that maybe you haven't been communicating things super clearly and you have been giving them 2nd and 3rd chances. I would personally follow your contract re: notice period but offer them their money back (if they paid in advance) if they leave right away.