Given it's only one link, the easiest solution is to contact the linking site, and ask them to remove it.
That said, a single bad link is unlikely to do much harm. The problems only arise when there are a large number of such links.
Regarding Google Webmaster Tools, you can use that to sumbit a disavow file, but you should be cautious about doing that. Google advises:
This is an advanced feature and should only be used with caution. If used incorrectly, this feature can potentially harm your site’s performance in Google’s search results. We recommend that you disavow backlinks only if you believe you have a considerable number of spammy, artificial, or low-quality links pointing to your site, and if you are confident that the links are causing issues for you. In most cases, Google can assess which links to trust without additional guidance, so most normal or typical sites will not need to use this tool.
More information here.