the easy way: tell them you're moving on in life and it no longer includes them.
the hard way: tell them every single detail you dislike about them and be as honest as possible (unless you want to lie then make up things that will truely hurt their feelings) and then walk away after telling them they've been dumped and tell them to deal with it.
Simple: Cheat on them, make sure they find out, and then they will do all the work.
I would NEVER do this myself as i do not think it is morally right, but you did ask for the easiest way.