Just get some ointment and rub it on there under the chin they cant lick it off but make sure whatever you put on you keep it clean you dont want stuff to stick to the ointment and cause and infection.
You cover the sore with a band-aid and neosporin. That should help the wound heal and keep the bunny from rubbing it more. If you have a way to create one of those plastic dog collars for the bunny, that should help.
