Replace whiteboard markers with flip chart markers.
First, realize that this will ruin the whiteboard forever. That said, since flip chart markers are nearly identical to whiteboard markers, the trap is easy to set. Ensure that all whiteboard markers are replaced. This plays upon the psychology of the one who grabs for a marker. If they all look the same, their identity will go unquestioned. How often do you check the markers before you use them?
Replacing the white board markers is an expensive prank to play. It may also serve as protest for offices that have SMART boards, who spent the money to acquire, but will not spend the time or money to train anyone to use it.
A note to whiteboard marker producers: can you really not get it together to make them more distinguishable? Triangular grip instead of round? Anything….anyone?
Here is a solution if you accidentally prank yourself: