To search engines Google, Yahoo!, Yandex and Bing, do not go through specific links and do not index them, and also to not pass on them PageRank, you need to add a special attribute to the link:
rel="nofollow"
For example:
<a href="example.com" rel="nofollow">Link text</a>
See also the information in this attribute in the Google Help Center – Attribute rel=”nofollow”
Information on Yandex – Using HTML tags