You can add a relation to multiple assets at once by clicking the Assets link at the top of the web console, ticking the checkboxes in front of the assets and hitting the Add Relations button on the left. There is no built-in functionality however to delete a relation from multiple assets at once. I've added the feature to the wish list, but don't have an immediate release date for it.
We can't really provide instructions or support for this, but you could remove the relations directly from the tblAssetRelations database table, e.g. with a script. We do recommend backing up your database prior to running any scripts, by following
these instructions.