If they are all paying you what you describe as "their fair share" then you have no control. If you didn't specify limits to begin with, then they all gain access as they see fit.
I'd be very careful getting into their system as you are not allowed to go there. Simply providing access to them, and collecting a fee for it, does not give you rights to their computer, you are making a mistake here.
If you want control over your bandwidth completely, then you need to disable their access across the board, once their fair share of funding is paid out, then re-enable them with limits, in writing, being very specific about the times they can or cannot operate. It's one thing to say you were very clear, it's something altogether different to state "Sign Here"
Just friendly advice, going off your own words and description of the situation.