So the CEO of the company pipes up and says, "What's to stop me from paying $20 to have someone to create 1,000s of spammy back-links to our competitor's site?"
I'm shocked that this hasn't become a more common or publicized tactic. I can't imagine how you'd trace it, and the way things are right now, all the burden of proof and cleanup is on the site owner.
They could certainly disavow the links, but it seems like you could pretty easily and cheaply become a pain in the ass of just about any small to medium site on the internet.