How can I block seeing someones tweets in Twinkle on the iPhone?

I have the Twinkle app, and when I go into the Nearby tab, I have it set to a radius of sometimes 50 or 100 miles. However there are some people that post rude and racist things, how can I block them? I notice if I click on their tweet to reply to them it starts with @ and then the username, but if I go to http://www.twitter.com/ that username , it cant find that person. Whats going on, how can I block those people?