In some cases competitors may attempt to lower you website value by creating hundreds or thousands of low quality links back to your site. In many cases it is next to impossible to know who is creating the links without going through a legal process to track and trace the original submissions.
Generally an easier way to deal with such issues is to contact the webmasters of these sites that are linking to your domain(s) or if that is not an option Google does offer a secret agent, for when the SEO task seems more like mission impossible you can use the "Disavow Tool" to remove all association from a link to your site. Doing this can be tedious and time consuming but very effective at removing negative links to your domain.
Abusing this tool can also get your domain flagged even more harshly than not, so always try other methods of removal before using this tool.