It is described in our Terms and Conditions page:

We disapprove websites which:

• Are placed on free domains/subdomains.
• Are not in the top 2 million sites ranked in Alexa.com.
• Have pages that have malicious code or illegal or adult content or links to such content.
• Promote or facilitate illegal or legally questionable activities such as pirating and hacking.
• Overlay or hide our ads, or overlay them with other ads; Our ad code must be well visible on the approved website and can not be put in other pages, scripts or frames.
• Place our ads on auto-refreshing pages.
• Contain materials of violence, obscene or vulgar language and abusive content.
• Display excessive number of ads or pop-unders.

Also, we are checking every click you get and count only clicks from good IPs. You could check the number of invalid clicks too from the page with your ad visitors.

We are considering an adult category though, as well as profiles for publishers from which they can choose which categories can be shown on their site, so expect that in the near future.