impede
     v 1: be a hindrance or obstacle to; "She is impeding the progress
          of our project" [syn: {hinder}]
     2: block passage through; "obstruct the path" [syn: {obstruct},
         {obturate}, {occlude}, {jam}, {block}, {close up}] [ant:
        {free}]
Source: WordNet® 2.0