WWF has already included a function that checks for the timespan between post you could use that function to restrict all database queries on your site, e.g. the search feature that is most critical regarding server overload.

 

You will still have the problem that somebody who writes a small tool that only calls your forum main page from different IPs would bring your server down because the latest version of WWF does a lot of database queries on the forums default page.

 

You could add a function to an included file (e.g. header.asp) that gets called on every page to check if a person is calling a page to often. Also you could restrict critical features like searching to members only so that guests do not have the chance to overload your server by doing repeated and simultaneous searches.

 

Just some ideas.