If you think they're being shady and you aren't afraid to walk in the grey/black hat arena, one thing you can do is negative SEO. -- Buy up some link blasts to the shittiest sites imaginably, or use a tool to do it manually. 20,000 new links in a month from porn/warez/etc sites to your competitor ought to take them out completely. The benefit here is there's really no way to trace it back to your site, and penalize your rankings because of this behavior. Edit: I'm not in SEO anymore, and I can say this is highly unethical, been about 5 years and I only did whitehat. I'm a developer now, and I don't condone or think negative SEO is proper, but it's done everyday, and I'm just laying it out as an option the op has.
I don't think negative SEO works anymore. Google devalues the inbound links, but doesn't penalize the target site. In the case of Rap Genius I think they made a change specifically for them that punished their bad behavior. I stepped out of an SEO project about 18 months ago so my info is probably dated. Google makes several updates a year.
