There are several ways to deal with this.
a) on ACS there is idle-timeout setting in the User Setup, if the user is idle for say 10 mins, ACS will declare it as disconnected.
b) if you do accounting, then a start and stop record is sent whenever a user connects or disconnects, and ACS keeps track that way.
Furthermore, you can configure max-session on ACS to whatever, if a session is already connected by a user, it will check if max-session is eg 1, then it won't allow the same user to connect again from elsewhere until the first is released.
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/csnt26/usergd26/ch3.htm#xtocid2899010
HTH
R/Yusuf