Login to your GAM account, the follow the steps below.
Add a new protection
Click the “New Protection” button (or edit an existing protection)
Apply any targeting, if you only want this protection to apply to specific inventory
Add URLs to block
Expand the “Advertiser URLs” section
Input each advertiser to be blocked on a separate line and click the “Add” button
Then click “Update”
Save
Finally, click the “Save” button
