Best answer by s l i g h t
Can you not remove it from the Metadata page when you edit the track?
If Artist Union had access to your account you need to make sure you revoke that.
Go to the Settings menu which is near the bottom of the black drop-down menu under the three dots on the top black menu bar. Then go to the bottom of the Accounts page where it lists Connected Applications. Revoke access to anything there that is not SoundCloud.com or Help Community.