Google officially recommends three ways to not pass
PR. Use of the nofollow directive is the newest way, but it is not necessarily the preferred one. It was developed as a counter to exploits arising from user generated content (forum spam for example). The two original methods to block the flow of page rank are: to use an interstitial page (that says "You are about to leave our page and go to xyz.com, its not our fault if they offend you...") which is blocked by robots.txt, or the use of a 302 redirect. The 302 redirect method is actually used by the search engines themselves. (all results in SERPs go through a 302 redirect on Google, Yahoo and MSN.)