you are not specific to your post. More explanation will be good if you want perfect solution. Any way if you are talking about filtering urls then you can go to your adsense account --> Adsense Setup --> Comptetitive Ad Filter. You can add upto 200 urls.
Its simple. Go to Configure...then to Filters and then add in the box the urls that you don´t want that will be displayed in you sites.
Do you mean block certain ads, that you don't like? if yes login to your google account. adsense setup then "Competitive Ad Filter" and then add url ads that you don't like and save