Search Engines are throwing 'illegal' SHOW requests?

Now that I've finally got all of my websites deployed I find interesting Exception_Notification messages being generated when the 'spiders' ask for illegal requests on my website. For example: I have a nested data structure whereas forums belongs to forcats. So a request through my application is http:\\www.websitename.com\forcats \1\forums\6. The search engines are throwing requests at me like: http:\\www.websitename.com\forums\6. My traditional SHOW action looks like this;

  def show     @forum = @forcat.forums.find(params[:id])     respond_to do |format|       format.html     end   end

Do you have any idea how I can protect against this? I am grateful for your reply. Kathleen