Have you tried turning the hose on him ?
seriously though, i would just tell him to go away. I'm quite blunt though, I always tell the girls next door to get lost, and last year they kept looking through the little holes in the fence, one day they were doing it when i was watering my plants so i squirted it at them mwhaha. They left me alone though
Oh dear, I fear we have this child! Micah shouts for next doors kids over the fence all the time and climbs over. The mum has said he's welcome anytime and that he can play on the trampoline, but he regularly wanders into their house and I have to climb over myself and fetch him back! To be fair their kids (3+5) come over to us too but they're easier to reason with because they're older than Micah.