There is no way negative SEO doesn't exist. As long as Google penalizes sites for link schemes those same schemes could be used against their competitors.
For example, say I'm running xrumer and spamming forums to increase my ranking, which was common pre-penguin. Google releases penguin and my sites rankings start to tank. So instead I switch to spamming my competitors links and Google penalizes them also. Good luck as a site owner trying to remove thousands of junk forum links especially when the forum owner thinks you were the spammer.
For example, say I'm running xrumer and spamming forums to increase my ranking, which was common pre-penguin. Google releases penguin and my sites rankings start to tank. So instead I switch to spamming my competitors links and Google penalizes them also. Good luck as a site owner trying to remove thousands of junk forum links especially when the forum owner thinks you were the spammer.