One of the most simple things that I know of is called a vinegar jar, and that can be used to sour relationships. You'll just need a small jar (baby food jar would be fine, they're about 50 cents in most stores), vinegar (also very cheap), some paper, a pencil. Rusted nails (or any nails, since they'll rust inside the jar), cat hair, dog hair, dirt (esp dirty the other guy has walked on recently is great). Fill it with nasty stuff, and shake that sucker up.
On the paper, you'd write the guy's name 3x down the paper, then turn it once to the right and write your command over it. You just focus your intent in that jar and shake it up a bunch.
That's the most simple, effective spell that I know of. It does require materials, but if you truly want it to work, you'll find a way to get them. Altogether, it shouldn't cost more than $5 for everything if you're anywhere in the US.