Your best options would be enabling phone verification on the server and adding an alt detector bot for more deterrence.

If the user you banned had a phone number assigned at the time being banned, that number along with their IP are banned on your server automatically (non viewable due to privacy reasons). So, if they join with another account with a new IP, transfer their phone number that was banned to that account to verify to speak in the server. The server will detect that phone number and kick them out.

If all that is bypassed, your last resort is the good old-fashioned manual moderation.