That thing is called the key. It keeps the impeller from spinning on the shaft.
Do you have 2 pry bars? They don't have to be big ones... just the typical 16" ers you can get at Home Depot.

Heat the center of the impeller with a propane torch. You don't have to get it real hot and I wouldn't because there's a plastic insert inside it. Place the 2 prybars across from each other and pry both sides simultaneously. The Impeller should come right off.