Just think about every technique you have tried to help you rankingthat got you sites deindexed or had their PR lowered and do that. Otherwise get a link spamming app like PRStorm or something like that and go and spam your competitors links to the spa*mhun*tress or Matt Cu*tts websites { I might be being paranoid but we dont want them here - that if if they arent already :0 }
Im sure you will get them banned in short order.
EDIT: I type too slow and didnt see Nick's post but I guess he is right. It all depends what you can do to people and still sleep soundly at night. Everybody's level is different. By the mere fact that you asking shows me that you could do this and still sleep soundly. I personally dont have to worry as there is still too many competitors in front of me in the SERPs to try and get them all banned