"Show me your friends and I'll show you your future" is as applicable to the internet as it is to offline life.
There are millions of potential advertisers out there. If this particular company doesn't fit your mold, pass them up. Chances are that if you're not too keen on their product, neither will the advertisers you'd want to deal with–and those are the kind of advertisers that are usually willing to pay a premium for relevant adspace.
When I first got into this business my younger self was willing to make any deal I could. Several months later, I was contacted by a major player. We went back and forth…worked out prices, etc. Then they saw that one "bad ad." No-go. :( And I had worked out an amount that was several times larger than my less-premium clients were paying for a similar space.
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I guess it is up to you, but don't me scared to turn someone down. Sometimes it is better to turn people away than to lock yourself into a relationship with a company for months. The worlds' most valuable and prestigious brands didn't get there by working with everyone. If the money can be levered into better advertising for your company, maybe it is worth the temporary "side-step."
After all this rambling, I guess what I want to say is this: make it worth your while. If accepting them as a customer means you want more money, tell them that. Some will get offended, the majority (the rest) will have already heard it 100 times.
P.S. If it is someone looking for links for "payday loans," you won't offend them no matter what you say. Ha! Those people get rejected more than anybody can even imagine.