Can you please explain in more detail the problem you are running into...and appropriate debug outputs. Is it a modem call (ISDN or CAS) or an ISDN call?
deb aaa authen / deb aaa author / deb radius (or tacacs) / deb ppp nego
Maybe you are trying the following...You can reject the caller or in other words limit the connection from the same caller to just one connection based on username or callerID of the caller.
Limiting to one session per username basis on aaa server.
