simultaneous ISDN and async

Can i use simultaneous use to limit particular users to use just one channel ISDN? And at the same time, to forbid async users multiple logins? For ISDN users i want to limit only some users, not all. I know that cisco questions should not be posted here and i am sorry for that but... I am not sure what can i do on NAS side to prevent double channel ISDN logins. Cisco is AS 3620, IOS is 12.0(7)T. If i can send some vendor specific packet to NAS that would be great. Thank you.