"haha at you."
The word "joo" is a typo of the word "You" and ever since someone made that mistake, joo became "you".
The @ simply represents "at", obviously.
The "Lol" means "laugh out loud"
Its sort of slang. A lot of scene kids say "Lol@joo". Look up scene if you don't know what scene is.
When somthing is really funny, people say it like "LOL@joo" or "LAWLS@JOO"
Eric: LOL@Joo? What does that mean?
Tommy: Uhm.. Go look it up?
Eric: -Looks it up- OHH OKAY. I get it now.
Tommy: Why did the chicken cross the road?
Eric: I don't know?
Tommy: To get to the other side.
Eric: OH. LOL@JOO.
Tommy: Dude. You're like, 35 years old. Don't say "LOL@JOO" you look like a teenager wanna be. Alright? kthnxbai.