"unit"? The unit has nothing to do with the CPC. You can filter out advertisers that you know (for some strange reason) are paying low in your account. But I see no point as advertisers come and go with time.. My suggestion is to look for some high paying keywords related to your niche and re-write your content after those.
Actually, if you use custom channels to compare the earnings of differently placed ads, sometimes there is one that consistent underperforms - it could be its position on the page, or just that there aren't enough good ads in your niche so that that particular ad unit gets the low paying ones. It doesn't hurt you at all to remove the code for that ad unit, and see how that changes your overall earnings. You can always put the code back later.