to the original question, you will probably have to use two allen wrenches to loosen both sides of the bolt you're dealing with. As one poster mentioned, it's probably got a sleeve and a bolt together in there. Don't be afraid to take it apart entirely to get a good look at what you're working with in there.

Next, be very sure to tighten stuff up nice and securely when you're done repositioning it all more to your liking. If you find yourself at all unsure about the safety of the stem or bars when you're done, take the bike to a local bike shop and ask for help. This adjustment could probably be done in about 3 minutes (and probably for free) if you ask nicely.

Don't underestimate the danger of incorrectly re-assembling/tightening your stem. Imagine your handlebars coming off while you're going 20 mph. Not good.