If you can stand reading a paragraph:
Someone in my family has been over for so f*** long and I want him to go the f*** away. He's gotten to the point where he stressed me past my limits as I am not used to people being nearby me for a long period of time.
Although I have very little power to tell him to piss off, how do I tell him that I'm stressed and I want him to go the hell away nicely?

Tl;dr:
I want someone to go away and I can't tell them to go the f*** away nicely.

Any suggestions?

How about in a way where people won't think I'm a complete a****** for practically telling someone to get the hell out?

People won't leave me alone about it.
"You told someone to go the f*** away? What kind of son are you?"
"That was mean. You didn't have to do that."
Etc. If people keep asking me about it, I'll get to the point where I explode on them too.

Good point there. When he pisses me off enough, I suppose I'll yell and show how much I want him to go the hell away. He's been here for a month of the Summer, after all.
I'll leave this open in case I get other suggestions.

Get one of those puzzles which you can hide something in for people to find when they complete it, and put a note in there describing to him exactly what you feel. Let him solve the puzzle.

get high with each other and tell him to f*** off

Just tell it to him straight. No beating around the bush. Don't shout, just say it like it was an everyday conversation. If you really want to cover your a**, preface with, "I'm gonna be honest with you, you really ... "

As long as they are invading your privacy or personal space, you have compelling reasons for them to back the f*** off.

Do you believe halving my computer time for a long time would cut it?
Tbh, if it's my computer, I should be the one using it IMO.

Hummmm. That's an awfully shallow thing to get uppity about, but I get where you're coming from. Is this relative younger or older than you?

Younger, but by merely half a year.

Then, without context, the best advice I can give you is to man up and swallow it unless he starts feeling entitled to your possessions- if that's the case, you're better off consulting your guardians than getting all uppity and aggressive. The important thing is to avoid any violent outbursts, 'cos under most circumstances it'll lead you down a very bad road.

Or you could just take the easy path and confront him directly and let him know exactly what you think of him... it's better to negotiate than to just react negatively

Kids, violence is never the answer.
~The More You Know

It sounds to me like it's a guest who's staying over for a short while
And if so, the correct thing to do would be to let your guest do whatever the hell he wants (within a reasonable margin of course)

But it sounds more like you're saying that you want more time on the computer (which is fairly selfish if you ask a younger relative of yours to "piss off")
I think you should reach a compromise like adding up the hours you use together on the computer, then halving it and making it completely fair

Well, the thing is, he's been here for about a month of the summer of which he honestly doesn't need to do.
I also do not trust him one bit. He does a few things only family should know, but he also does s*** such as steal accounts on online games and attempt to get hacks for RuneScape.
I'd say pornography is a problem, but I'm guilty of that myself.