I'm not a serious ISA guru, but I will refer you to the forums on www.isaserver.org, which is run by Tom Shinder who is an absolute authority on the matter. I know that for Windows connectivity generally, you can use the cconnect.exe utility from the 2K resource kit to limit users to a single concurrent connection.
However, from a security standpoint you should also take non-technical steps to try to alleviate the "users sharing passwords" problem, because it is a huge breach of security on your network. It's a good step to figure out how to limit your users to a single connection, but the larger problem (to me, at least) is stopping your users from sharing their passwords in the first place. Depending on the current configuration of your account and password policies, I would begin mandating more frequent password changes and account lockouts to drive home the point.
This was first published in April 2004