You could use Disavow option in your webmaster tools if you have purchased such low quality links. You could also delete such links through your signatures manually.
Google Webmaster Disavow tool is best tool Remove the Low Pr Backlinks of website.
When and How to Use the Google Disavow Tool
The higher the number of relevant sites linking to you, the higher your rankings will be, right? What happens if a non-relevant site keeps linking to you? Or worse, you get a backlink from an adult or gambling site?
These irrelevant links can be hurting your rankings. For this reason, Google released the Disavow Tool as a way for you to tell Google to not count specific backlinks.
These are few useful tools for remove spammy & low quality links from your site-
LinkDelete, Open Site Explorer, Rmoov, Ahrefs, Linkquidator, MajesticSEO, Google Webmaster Tools, Elbow Grease, Remove’em.