This has been discussed many times. Yes, you actually can get banned for it and some here say they have. At 94% though, you SHOULDN'T have to worry, but I have heard crazier things...
Besides, there's really no use sending all traffic to pages with YPN ads when you aren't even going to get paid for it if the visitor isn't from the USA.
If you are that worried you might want to try out MaxMind or phpadsnew, both have Geotargetting capibilities. Although, I believe the success rate for them is only 95% (it's most likely even less than that), so I'm not sure how much that would help.
thank you garyr_h for your comment. so if I use any of those scripts that you recommend me it wont help much since they are not 100% accurate. so I guess my only otion is to hope and pray they wont ban my account :(
but I mean, I dont think there is a single website in this planet with 100% us traffic, so lets hope they review the sites manually and dont leave the work to a bot