Decline any crap. MFA, pill sites, porn etc. Sites without unique content, a design that makes it easy enough to read should be kept in mind.
You need to identify target audience for your directory and accordingly publish TOS so that no one attempts to submit links which do not qualify. Of course pills, ciallis, viagra etc are generally spam links and should be deleted / prevented even from submitting ! Your directory looks good - keep it up! Good luck
Sites that use adsense are not a problem. For example I'd happily list digg, DP and ezine-articles. However sites that scrape content from other sites, scrape youtube etc and are riddled with Adsense ads you shouldn't approve.
I don't know how you could force an people to accept an website in the directory.If you decide to not accept an website that doesn't comply, you can choose to refund the payment. I don't see any problem with that.
Read and understand the terms of any advertising system you want to use, because there's a lot more to it than just banning porn. Also take a look at Paypal's terms, in case you want to use them in future. They're very strict (although sometimes inconsistent) on gambling websites. Another thing to look out for is viruses and other malware. Your antivirus won't necessarily catch it all, so make a habit of doing third-party checks: Google the site in question, in case of "This site may harm your computer" warnings, or check it out in McAfee's SiteAdvisor.