disconnect a session

Hi We use fortigate captive portal with radius authentication. We need to disconnect a user or reauthorize a user after he has specific amount of download. 1- Is there any way to disconnect a connected user programatically? I have seen the use of radius " Packet of disconnect" or " change of authorization" extentions or reauthorization in specific time intervals. But I guess fortigate does not support any of them. 2- is there anyway to change the assigned group for a connected user on fly? thanks