Providing you manage a genuine whitehat site (and it sounds like you do) this should put your mind at rest:
Can Paid Links Be Used as Weapons in the SERPs?
The short answer is “almost never.” But, as is typical in the SEO world, there’s a lot more in the long version.
In general, it’s very, very hard to bring down a white hat site/page ranking well in the search results. Although Google isn’t perfect at catching spam, they seem to be surprisingly excellent (almost prescient) at detecting the intent of links. My suspicion is that sites who buy links to prop up their own rankings have very different patterns than those who have competitors buying links to them. These patterns exist on the sites themselves, in other sites registered to the owners, in link footprints and in usage/search behavior.
https://moz.com/blog/what-if-my-competitors-point-spammy-links-to-my-site
Also there is a disavow tool from Google Webmasters:
Disavow links
If you believe your site’s ranking is being harmed by low-quality links you do not control, you can ask Google not to take them into account when assessing your site. You should still make every effort to clean up unnatural links pointing to your site. Simply disavowing them isn’t enough.
And this advice sounds prudent:
Although you cannot stop someone from deliberately targeting your website with spam links, it is possible to fight back. There is no need to spend huge sums of money, or waste countless hours building thousands of positive links. Instead, you need to focus on adding more quality content to your website. Google is looking for websites that offer sublime content, rather than sites that simply use content to attract ad clicks. In addition to adding quality content, you should monitor your Google Webmaster Tools account for suspicious links that happen all of a sudden.
Negative SEO sounds sinister.
Good luck!
