I've seen some venues away from they real place on Foursquare. (2-3 km away actually) Is there a way to correct these positions without claiming the venues? (Preferably from the web interface)
You can flag a venue as closed, duplicate, or as having an incorrect address from the app. On the Android click on menu and then Edit Venue when on a venue page. On the iPhone, click on the square with the arrow in the upper right corner to flag a venue as a duplicate. A superuser will approve your suggested merge. If you're a superuser, you can suggest merges from the Edit Venue view, and go through the merge queue for your town.
You can also report duplicate venues on your computer at http://getsatisfaction.com/foursquare where our superusers will help you out!
After some search I've found my answer.
Foursquare support says that it is possible to edit venues on the web but to do that you have to be a superuser, the mayor or the one who added that venue.