You can “delete” a watcher under Admin > Users > Select.
Search for the desired user and then uncheck “Let this user access RT” and Save the update.
Users are never really deleted in RT but this should hide it from normal use.
Alternatively, you could update the Username to something shorter and friendlier. Users “Autocreated when added as a watcher” by RT usually set the Username to the same as the email address but it will prefer Username if it is available when displaying names.