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Even though i think you're a bit of a douche for attempting this, the easiest way i can think of would be to disable DHCP on the router completely. Then change the ip address of the router to another subnet, like 10.10.10.1 255.255.255.0, and assign yourself a static IP in the same subnet.
Any PC without a static IP in that subnet would have zero network access. Then change the router back to the normal network and re-enable dhcp when you're finished.
That being said, its still a terrible idea, if he notices his internet isn't working first thing he'll do is go reset the router and fuck you over anyways.
edit: this is a pretty risky step though, if you don't really know what you're doing you stand a good chance of locking yourself out of the router too... then you'll have to go explain why you need to hit the factory reset button on it.
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