A website can direct a search engine spider not to follow a link that appears on it using no-follow command.
nofollow is a non-standard HTML attribute value used to instruct search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of spamdexing, thereby improving the quality of search engine results and preventing spamdexing from occurring in the first place.