You can get a registration form for a service by typing M-x jabber-get-register and entering the JID of the service. On success, you get a single-stage form to fill in.
There are two buttons at the bottom of the form, “Submit” and “Cancel registration”. “Submit” does what you would expect it to, but “Cancel registration” cancels any existing registration with the service. Whichever of them you choose, you get a message in the echo area informing whether the operation succeeded.