I'm in a job where I spend a lot of time in other people's conference rooms. If they're cunts I leave a couple of permanent markers in their whiteboard tray.

@jon @fesshole who's more affected : the cleaning team who probably have the right solvent on hand, or the people having a meeting who suddenly realise their passing thoughts are indelible

(I use "permanent" marker as a durable marker on my own stuff - on many surfaces it cleanly wipes off with isopropanol!)