While creating or editing a campaign go to the Traffic Sources tab.
By default all campaigns target RON, which doesn't include any adult traffic. The placements included in your targeting selection appear on the right.
In order to not receive traffic from specific categories move to the Blacklist subtab, click on Categories and tick in any check-box. The blacklisted placements are displayed in the list on the right.
If you don't want to run an RON campaign, instead, untick the checkbox and manually select the placements or categories you want to target.