block users on-the-fly

Hi, Does anyone already have a program to block freeradius on-the-fly? ie: user has PAID = YES in radcheck table. Whenever I set PAID = NO, the user would no longer authenticate the next time he/ she logs in. OK, this works, but, if the user is already loged in, even if I set PAID = NO, the user would not be rejected (for obvious reasons). This is important because the grand number of Router mode ADSL users, that never logs out. I'm building a program to verify every x minutes the database and if PAID = NO, return a flag to freeradius and then reject the user. Is there any other means to do that? Thanks.