is there any solution for this

Hi all

   i am facing a very irritating and unique problem and bcoz of this we are under serious problems. the problem which we are facing is like this.     

    first let me brief you with the details of our setup.  

    we are broadband ISP , located in india, we provide internet services over HFC(hybrid-fiber-coax) network , our customers gets connected using the motorol surf board cable modem. we are using cisco uBR 7246 CMTS and interlink radius for
user-authentication.

    nwe have many cyber cafe customers , we sell account to cyber cafe depending on the no of PC's the cafe have and we provide those many no of user-id & password , i.e if say cyber cafe x has N no of PC's then we provide them with "N" no of Uid-Password, and we are not doing a time or usage based billing for cyber cafe's. right now the MAX Simultanius-session is "3". so the user can connect from 3 different PC's simultaneously using the same Login/password.

    now the actual problem is these cyber cafe customer's have started celling the accounts to end users, as they can login from 3 diff m/c at the same time, or if we restrict thi to 1 simultaneous session then what they are doing is if they have 10 pc's then they are taking connections for 20pc's and then inturn selling those 10 connection to other end user's,

    so now none of our existing customer's are renewing their account and they are directly buying it from this cyber cafe's.

   they only way i feel i can restrict this is by associating cable modem's HFC MAC ID or cable modem's IP address with the login name of cyber cafe's.but i have no idea whether this is possible or not ,

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   and as i am having very less knowledge in this area i need inputs from you guys who definately hav more experience then me.

  kindly guide me , and give me your inputs that how can i handle this thing.

eagerly waiting for your inputs

regards
Prashant