[Samba] do not allow users to login twice

[oeh univie edv lists]

Hello,

I plan to have roaming profiles and I do not want users to login twice
with the same account on different work stations because then I guess
there could be problems of synchronising back to the server and possible
loss of data... 

What would be the best way to allow one user to login just on one computer
and not on more computers simultaneously? (They should be able to login
everywhere so I do not want to restrict users to certain host
names/netbios names)

kind regards, birgit