are you talking about adwords? and if so, search or content network?
if you're talking adwords search:
if you get a lot of impressions and zero clicks you should start by testing your ads before you delete the keyword from the campaign. come up with better ad copy or take a good look at the guys that are consistently in search results and do something similar. if you have time you can tweak the ad copy for each keyword (if they're in separate adgroups) and push ctrs through the roof.
if you're talking content:
you can run Keyword & Placement reports in the 'Reports' tab in adwords and see exactly which sites are giving tons of impressions and no clicks. find those and add them as negative sites in your Campaign Settings.
if you can get a little more specific with your question I'm sure you'll get a more specific answer.