So google says there is nothing a competitor can do to hurt your rankings. This is true to a certain extent unless they are getting links from questionable places. This is what you do: find the sites you want to target and do some backlink research on them to see where their links are coming from. If the links look like paid links then you can report it to google. Google will then investigate it and then if they see something that is questionable then they will invalidate the links (ie. treat them as no follow). I recently did this to a competitor who was on the page 1 results for months. I reported the links last month and I just checked the keyword again. The competitor dropped to the 2nd page(around rank 18-20)! How funny is that? Then again the links I reported was from a pr 5 site that has 10s of thousands of pages. Their link was sitewide. Ouch. Should be interesting to see how google treats that site from now on seeing as how I reported paid links pointing to the site lol.