Sounds like a dick. He’s not your friend, he doesn’t like you.
Just stay away from him, if he’s engrained with a certain group of friends/ social circle find a new one, there are literally thousands of new people to meet at Uni.
Sounds like you’re in halls?
It doesn’t feel like it now but by throwing piss on your door he’s given you a lot of leverage over him.
If he does anything again or even harasses you report him to the warden and I’m 90% sure they will move him to another part of the building and tell him to stay the fuck away or even chuck him out. Most UK unis are big on pastoral care and will find this unacceptable and strongly side with you.
Tommorow tell your tutor or a senior lecturer etc that you get on with about this. Phrase it as asking them for advice , maybe tart it up and say its affecting your studies, concentration etc. This is so you now have an official on your side who you can use as a reference if he messes with you again. They also might actually give you some good advice .
As some general advice force yourself to be more social, if clubs and bars are way out of your comfort zone then thats where you should be going. Get involved in some societies but not geeky ones, stuff like team sports, Rugby comes to mind it and will toughen you up socially and mentally as well as physically.
Uni should be about getting a good degree, developing your social skills and getting as much young skirt as you can handle. Focus on these things and cut off that which holds you back.