Prevent ( disable ) some users from sending / receiving hir / her emails...
Including via WebMail System ( http://www.horde/imp ).

Fastest, easiest and quickly reversible - change their password.

Sorry, do you know "What is disable" ?

Maybe you need to be more precise?

'Disable' can mean many things... do you want to REJECT all new/incoming mail for this user? Then you have to do that on your SMTP server side, otherwise, incoming mail will still be delivered to their mailbox.

If you simply want to prevent a user from being able to view their existing email through imap/dovecot - which, by the way, if you go bnack and re-read your original post, is what you asked for - (this would apply to any/all imap clients, including webmail), and from sending through your SMTP server, then - *assuming that you are using the same password backend for both* - simply changing the password is the best way to go, as Bart said. I do this all the time - it is a very effective way to get someone's attention. ;)

Of course, if you are not using the same password backend for imap and smtp - and if this is so, then I'd have to ask *why not!* - you'd have to change both.