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:\\\forcats
The search engines are throwing requests at me like:
My traditional SHOW action looks like this;

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

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