Cant help you with your other questions, but...

> How can I delete users ? Is it enough to delete the lines of the user
> in /etc/passwd and master.passwd or do I have to do more ?

Easiest way is to use 'vipw' whne logging in properly as root - as
long as you know how to use vi anyway.  If not, use the 'hjkl' keys 
to navigate to the lines you want to delete and then 'dd' over that 
line (removes whole line).  As soon as you exit vipw (press : and 
then type 'wq') the password file will be rebuild.

Thnking about this, you probably already know how to use vi if you 
created the users in the first place, but one nevers knows....

All the best,

  Duncs
-- 
+-------------------------------------------------------+
| Duncan Ferguson               Network Support Officer |
|         Worcester College of Higher Education         |
+=======================================================+
| My views are not necessarily supported by the college |
|                And who can blame them?                |
+-------------------------------------------------------+