In that case, take out the little ink pipe from a Biro and discard it, then keep the empty pen in your back pocket.
When he next goes to the stationary cupboard follow him in, shut the door behind you and stab him repeatedly in the throat. Leave the pen there for all to see and exit the cupboard in tears, claiming that you found him choking on a pencil eraser and you were trying to save him with an emergency tracheotomy.
Ha! Amazing long-term strategeering there Lucy!