You can switch to the HTML view, find the relevant link and add this before the close bracket:

rel="nofollow"

Here's an example:

<a href="http://www.domain.com/page.html" rel="nofollow">Anchor text</a>

Cheers,

Martin.

Thank you Hong Kong Dragon

here's an WordPress Codex info on this topic..
Nofollow « WordPress Codex