[Samba] Prohibit Password Change on specific user

[Ulrich Schinzli]

Hi There,

Setup:
- Debian Squeeze
- Samba 3.5.6
- passdb-backend: tdbsam

I have some machines running here, which are used by students. There is 
a "global" account for all of them. I'd like to prevent them from 
changing the password, so that nobody can make it a exclusive account. 
Is ther a way to do that?
Is it possible to do that with pdbedit?

Thanks for help in advance

regards
uli