if you have a girlfriend thats would be really akward

I think this is sort of hilarious and has great potential for more shenanigans.

Two ways to approach this: call the cops for harrasment, or give him a taste of his own medicine and print rude things through his printer.

If you're in an apt, you should post them around the building with his apt number on them.

See if you can set up a login code for your printer to avoid this happening in the future. If necessary call the manufacturer and they should be able to talk you through it.

you should leave it on his door with a note saying is your pinky or your penis?

Your network needs a password

Best prank ever

How do you know its his dick?

That is hilarious. A world of opportunity just opened up to you.