With the help of lock.tcl, you, as admin, are able to lock special settings for user. Either you lock them globally (you will have to edit lock.tcl for this - line 3: set ::globallocks), or for a special user. As admin, you can use the following commands:
/sbnc lock <user> <setting> /sbnc unlock <user> <setting>
If a setting is locked, the user is not able to change them, neither using /sbnc set nor using the webinterface (it wont be shown there at all).
Of course, you can set and unset locks via webinterface too (userlist → click on a user).
Note that this will work for awaymessage too (in combination with awaymessage.tcl)