Roughly speaking, I try to keep the news and editorial page department separate from the advertising department. Obviously, they're both me, so that doesn't necessarily work perfectly. But, in practice it means I'll take ads from anything that doesn't appear to be overly offensive or illegal. In terms of ad placement, I make no promises about which ads get put on top/bottom. Sometimes I rotate them randomly, sometimes I do "most recent ad on top" and sometimes I just forget about it and don't pay any attention to it.
I don't make any promises to any advertisers that a paid ad buys you positive coverage. In fact, I hope that I can make a promise to the readers that the ads won't influence my coverage at all. For most ads that isn't relevant, but for some it is. Prospective advertisers should recognize that a wall does exist between the editorial board and the marketing department.
Another thing - I probably have more ads than I should which means two thing. First, I'm raising my prices a bit. And, second, I'm going to start capping the number of ads at a certain number, once I figure out what the number should be. Obviously if the number of ads declines, the prices will come back down. Supply and Demand and all that.
....UPDATE: Josh has clarified his policy, and indeed he had just meant to say he wouldn't let one candidate by all the adspace.