I've submitted this good idea as a bug here:
http://bugzilla.zimbra.com/show_bug.cgi?id=16099
I find this a good point for security, because some users should
be restricted to IMAP/POP access, and Zimbra Webclient could be
an extra feature.
Or for some paranoic boss it could be a security related issue
to lock the users to a specific kind of access (users in the
office only IMAP, users on the roadonly webclient)
As addition a feature for disabling users to delete emails
(or onlydeny empty trash) could be fine for audit issues.