Got the Runs Brad, a very popular, expert bartender at the busiest seafood chain in town, recommends this handy revenge technique:
When he has a patron who is behaving badly or tipping poorly, or both, he waits until the offender orders another drink. Then he swishes the glass into the bar cleanser sink, leaving a light coating of the nasty soap-like chemical on the glass. In the case of beer, the chemical results in a nice foamy head and MAJOR bowel and stomach problems a few hours later. Rob claims he has actually seen patrons run to the restroom.