years ago, my ex and I went to a halloween party, everyone had used the restroom with no problem till my ex went in(I didn't have to go).
he came out frustrated because the more he washed his hands, the blacker they got. our host announced to everyone that he was the only one of the whole party who washed his hands in the rest room.She had planted a gag bar of soap to fit the theme of the party and never expected only one person to have been pranked. there was much embarrassment in the room and everyone went to the kitchen sink to wash their paws.
maybe you need gag soap and wait at the restroom door with a sign saying you didn't wash your hands!