Blacklisting, or shutting down placements that aren't working well for your campaign is very easy to do.
Simply edit the relevant campaign and go to the targeting page,
Enter the publishers ID in the SubID text box and choose if you want to Exclude these placements (Blacklist) or Include these placements (Whitelist).
Click save, those placements are now blacklisted/whitelisted on your campaign.
For any further assistance, please contact us through the chat on your account.