internet access

i have another problem….i restrict internet access by ip #’s…i set up the ones with access to use certain ip #’s so they can access and the other pc’s get assigned a different ip..my problem comes when a pc (windows 2000 pro) has several users assigned to it and only one user has internet rights..the other users are not allowed access..how can i restrict them from getting on the internet…i did take away there permissions on the iexplore.exe so they cannot click internet explorer and get on, but i have found out they email themselves a web link that will then let them access the internet…how can i restrict a pc with internet rights but restrict only certain users of that pc from getting on the internet?
thanks