To stop crawlers (robots) from crawling and indexing iframe & img tags you can use robots.txt or meta tag!
rel="nofollow" attribute is used in links (hyperlink) related with <a> tag which may decrease your PageRank, this is a common practice used to fight link spamming.
Robots <META> tag
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
You can also use more aggressive approach!
Disallow: URL or a subdirectory goes here
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