restrict simultaneous logins

[Per Mathisen]

At the university here we are having the problem that some students bring 
several non-student friends to our computer labs, login with the
student's username on several computers, and sit there playing games or 
whatnot, to the great annoyance of the students who try to do some work.
We would like therefore to configure samba somehow to accept only one
simultaneous domain logon for each user. Is this possible?

Yours,
Per I Mathisen