I just did it with an old mountain bike frame, just set the rear derailleur high limit screw to where it holds the chain on the gear you want. You can remove both the front and rear cables, and the front derailleur. This setup has worked flawlessly for me. Just remember, keep your chainline as straight as possible to avoid having the chain come off. If you need more explaination, go to

www.sheldonbrown.com and look for the single speed link, it has some great articles and tips for setting up a single speed.