Joined: Feb 2012
Posts: 19,361
Likes: 5,500
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Have you confirmed that the wedge deep inside the steerer is free? Usually the wedge bolt (the really long bolt running inside the stem and steerer) is loosened a few turns then a hammer strike on the bolt's head breaks loose the wedge. Only then can you remove the stem if all is right. If the stem is frozen in the steerer from galvanic corrosion then the wedge will be freely rattling around and yet still the stem is stuck.
If this is the case then search for "stuck seat post" threads as this is the more common situation of stuck Alu in steel tubes. If worse come to be then a SawZaw or hack saw slice down the stem's quill will split it and let it come free. But try the chemical attempts first. Andy