If you want to do this without hacking anything...
AdminCP > Forums and Moderators > Forum Manager > [Forum you wish to hide] > Forum is Active
If you set that to no, it is still accessible, but won't be available on the main forum index. If you wanted to hide it further, you could edit the template for threads within that forum so as to not display the parent forum within the navbar/navbits or even redirect hits on that forum to somewhere else so you can't even access it manually.