Recently, I was asked to find a way to delete the 'Where From' link in the Get Info box of the Finder, because the info key was too incriminating. I didn't want to download any software, so I set about looking in the file's metadata. After running mdls and xattr on the file, I finally hit upon a way to delete the key.
If you run xattr on your downloaded file, like this:
You'll get a key named com.apple.metadata:kMDItemWhereFroms. To delete this key in the Terminal, run:
xattr -d com.apple.metadata:kMDItemWhereFroms /Path/To/File
I knew I would have to do this often, so instead of running a shell script every time I wanted to strip the Where From, I wrote an AppleScript:
Mac OS X Hints