There are some corkers on here! Mine is pretty tame in comparison.
When I lived in a shared house, one housemate was the messiest slob ever. Plates of half-eaten food growing fur under his bed, milk turning to cottage cheese in the fridge... he once used my clean tea towel to wipe mud off his shoes. Less offensive, but no less annoying was his habit of hanging his laundry on the radiators to dry, and leaving it there for months. Seriously, some of his pants were dusty, they'd been there so long.
He didn't respond to hints and polite requests to clean his shit up, and nobody had the guts to give him a stern talking to. So one night I got up after he'd gone to bed, got all his socks off the radiators, sewed the tops up, and put them back.
I still chuckle when I remember his puzzled expression, wondering why his socks wouldn't open.
It was a happy outcome. He laughed when he found out what I'd done, and never left his washing lying around again (still couldn't cure him of other disgusting habits though). Also, as revenge on my revenge, he waited until I was in the bath, snuck into my room and sewed my curtains together. Much hilarity ensued.