Awww....my heart breaks for this poor kid. He just wants someone to accept him and be his friend and you're looking for ways to reject him. This sounds very, very high school. I guess when you get older you're less judgmental towards people. What is so terribly annoying about this guy?
Why don't you take him under your wing and cultivate his better qualities?
I bet half his issue is insecurity and being rejected by people (that's why he tries so hard). Some of my best friends came out of people who annoyed me a bit at first.
Otherwise, I say grow a spine and be direct. Outright tell him he is not invited when you make plans.
Your current approach is obviously not clever or effective.
You don't need to be clever or effective to make someone gtfo. Plus, I'm the kind of INTP who prefers to be direct and avoid bullshit. If it's not important, it's not worth my time which could be used for other things...like slacking. I was direct with this kid, but I wasn't trying to offend him and just say, "Dude you act like a little middle schooler, now gtfo."
The best metaphor I can ascribe to this kid is that he's like a little gay toy poodle who's constantly trying to hump other dogs, which ignore him.
The dude's a fucking creeper, for instance, he followed my one friend everywhere, even to the bathroom while he was trying to brush his teeth and shit. He'll just stand there and watch people, and it's fucking creepy. He also makes annoying sounds and asks the same fucking questions over and over even after you've given him a proper response. Example:
"Hey dude, can I borrow ten dollars?"
-"No."
"Oh...So can I borrow ten dollars?"
-"I'm not giving you ten dollars."
And the way the conversation continues, he's not even trying to be an annoying little bastard, he just is one. When people have told him he's not invited to things, he'll just show up anyway, or wait outside like a lost puppy.
My approach has been effective because he's stopped bothering me, but he still bothers my other friends. As for being "high school," I'm generally pretty open and forgiving, but this kid just gets under the skin with his creepiness and annoyance. He truly acts like a middle schooler. If anything, I could insinuate that you're just being too motherly and optimistic, believing that we should "take him under our wing." Seriously? Get your head out of the clouds, that's not how people operate. It's not my M.O. and I would need to sort out the problems in my life before I would work on anyone else's.