SBR support CoA/DM.You can control sessions using SBR Administrator, a command line utility, or you can develop your own client management application to interface with the Steel-Belted Radius Carrier CoA/DM XML interface.
I have seen this working in a few environments. Are you trying to implement this? Having trouble?
Take a look at the SessionControl module in the admin guide for more information on how to configure the module.
One more thing: you woulld need additional license to use this module in your environment.
And noe more thing: consider using SRC in your network. It has Dynamic Service Activator (DSA) embedded component which allows you achieve same goals (activate/deactivate specific sessions on demand).
It also supports pre-paid scenarios using Volume Tracking Applicatino component.
Hope this helps