check any suspect URLS in your ad boxes against keywordspy.com and see the general interest of the company. I have a real estate blog that would randomly have ad links which point to yellowpages and were obviously very low cpc I would check the URL against keywordspy and see their targets and avg cpc and eliminate low bid advertisers.
Google is designed to show your site the ads people reading it are most likely to click on. That one that didn't pay much? Blocking it might've not gotten you a click at all. Use the competitive filter if one particular low-paying ad really bothers you - I do for everything about piracy & "free smilies" type download sites.