I cannot.

If I go to the file and click Sharing, I can see the name of the person I have shared it with.

If I click that person's name, it shows their profile, with 'Can edit' below. But below that, is just a button saying 'Close'. I cannot see any buttons saying 'Permissions', nor can I simply uncheck the 'Can edit' box.

Can anyone help? I'm not going to use Skydrive to share things with people if I cannot then unshare them.

This is all via the Skydrive browser app, through Chrome, in Vista.