Sometimes an SAE Allen key will be just a bit bigger than the equivalent metric Allen key. Use this to get enough bite to loosen the quill bolt. If not, and if the bolt isn't countersunk (flush) you might try something like those left handed destructive removal sockets you can get from craftsman or harbor freight.
Thanks for the quick reply. Any idea from the make/model what size stem it is?
Zoom stems came in 1", 1 1/8" and 1 1/4" so I can't necessarily tell by make alone. Looks like a 1 1/8" to me, but I'd find someone to confirm. If you can't get a 32mm headset wrench on the locknut, it's larger than 1".