Yea, take Doug's suggestion, use the largest deep well socket that fits inside the steerer tube and tap it down with that, I can't remember the size right now but works like a charm and you don't have to worry about stripping threads. Also, easier than taking a saw to the steerer tube to cut down, use a tube cutter that one would use on copper pipe for plumbing purposes, gives a nice even clean cut, if the steerer is steel, it will take several revolutions more than aluminum but don't use on carbon steerers.