xattr -d com.apple.metadata:kMDItemWhereFroms /Path/To/File
Alternatively, instead of running a shell script every time you can write an AppleScript:
on deleteWhereFrom(fileToClean)
try
set posixPath to quoted form of POSIX path of fileToClean
do shell script "xattr -d com.apple.metadata:kMDItemWhereFroms " & posixPath
end try
end deleteWhereFrom
on open imgs
repeat with i in imgs
deleteWhereFrom(i)
end repeat
end open
on run
set imgs to choose file with multiple selections allowed
repeat with i in imgs
deleteWhereFrom(i)
end repeat
end run
This can then be saved to the desktop as an application. Now, simply drag and drop as a
batch operation to delete the metadata key.