I don't think a mallet is going to do the business, you need a real hammer and a brass or aluminium block between the hammer and the top of the shaft, so you don't bruise it. You need to support the deck solidly (onto concrete) in multiple places as close as possible to the outside of the cutter disk. I'd use my cut-down one-handed sledge. The trick is to not hit it any harder than necessary, but use a very rigid setup and lots of hammer mass. I'd start by just dropping the little sledge about 18" onto it. If it doesn't move, look for ways to support the deck closer in, nearer the shaft, don't just keep hitting it or hit it harder. Remember, the closer-in and more rigidly you support the deck, the greater the shock transmitted and the more effective it will be. Don't fall for the trap of pounding the thing repeatedly with poor support - you'll just ruin the deck without achieving anything much (except for destroying the spindle bearings).
