Publishers may reject or refund an ad at any time, and for any reason.
To help avoid this, make sure to read the "zoneRules" listed in the site profile when you buy your ad and ensure you are placing ads that match the content level of the website. Please keep in mind, just because you comply with the "zoneRules" does not mean your ad cannot be rejected. A publisher may reject your ad simply because they do not "like" it.
The Advertiser will receive a refund for the unused time, and the Publisher will be paid for the time the ad ran. You may re-buy any Ad as long as you follow the Publisher's rules, requests, or specific reasons for rejection.
Publishers may also block any RON campaigns of which they do not approve.
Here is an example of a site profile with specific zoneRules outlined: