Prevent the pages which link to you from being indexed... Though it's not a good idea. :p

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I see, but adding nofollow would harm us in ranking ? I think member here Angela has done something to her site and if we check her site in backlinkwatch.com we don't see backlinks.

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By the way why you want to hide them, i mean are you afraid of competitor or what :-S

The reason Angela's backlinks don't show in tools like Y!SE is because many of the profile pages she links from do not get indexed. The search engines will still see most of these pages and recognise the links offered (and these do help ranking in my experience), but won't index the pages unless a few links are pointing their way.
Forum posts are almost always indexed due to regular crawling.

If you don't want links to show, try the process I am about to describe (have not tested, but it may work): point all backlinks to http://domain.com BUT specify http://www.domain.com as the preferred version of your site in htaccess (or using WP's settings) and Google Webmaster Tools (the latter probably unnecessary).
I find that Y!SE shows only backlinks for the EXACT URL you type in. So typing in the non-www version brings up different backlinks than it does for the www version.

So if you competitors simply copy and paste your URL into Y!SE (or use a tool that makes use of the Y!SE API for backlinks - most do), you should be fine.

However: I think that spreading backlinks between the www/non-www versions of your site is not very effective. Search engines may not attribute all links to the page that you're trying to rank.

This all really means nothing, though. Why on Earth do you want to hide your backlinks?

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