host the sham affair and make it a day they will regret forever... tell them in advance you do not want their business and they will not be happy with your services and advise them to go where they will feel more wanted
then be rude to the attendees, if catering, make sure anything usually cold is warm and vise verse for things warm, spread manure the day before, etc,etc,etc...
there's no law against bad service specially since they were warned in advance