Boardhost.com offers many plan packages. The basic is about $9.99 per month and provides ad free message boards of up to 500 messages. If your site receives over 500 posts the older ones are deleted to keep the forum under the 500 post allotment. Their other plan packages offer higher message limits and/or the option of more boards.
Their forums include a long list of useful forum features. Posters can submit messages in HTML or through an easy to use text editor. They can put smilies in their messages as well as share polls, receive email notifications, search posts and they can run spell checks on their text.
Boardhost forums offer numerous administrator tools including board customization, password and access settings, thread customization, IP banning and much more. To help prevent spam posting you can set spam flood limits so that users cannot post more than one message within a specific period of time.
This is a pretty good forum service; however, we would like to see more security features like spam prevention ability and HTML filters.