Actually i don't see any reason for retaliating by cutting into their lane closely and purposely slowing them down. I mean two wrongs don't make one right. As you mentioned, he's already being a moron by driving slowly in the fast lane, why do you wanna be like him by slowing him down and trying to cause unnecessary trouble by cutting into his lane abruptly.
I've been in situations whereby the slower car in front doesn't wanna give way. Whether he does it by purpose or he's plain ignorant or in the handphone, it doesn't matter to me coz i just wanna go faster. I'll overtake him from the left and just speed off. No point giving him the stare, the finger or cut into his lane abruptly coz my objective is just to reach my destination earlier, instead getting stuck behind him for the next 10km. In fact, most of the time when i meet road hogger, they're just old folks who're not used to the fast paced driving nowadays or even P drivers or quite simply ignorant drivers who just don't give way to others.