Never happened to me, but if it did I wouldn't want to hammer on the pump. Tapping while holding on to it with the other hand yes, but not much more than that. Remember that even though you don't care about the pump that's on there now, there is also the mating surface on the block to consider. It's easy to imagine the pump impeller making solid contact with the block if it's being driven out with lots of force. If you put any kind of scratch into that surface, that's likely to be really bad news. The guy in the other thread did manage to cut/drill/hammer it out, but IIRC that Aveo never got back on the road to confirm if there was any damage caused by the stuff he did to get it out.

If tapping doesn't break it free, you might try this. I have a large slip joint pliars which is a really great tool to have if you don't already have one. Mine opens to 4 1/2" and has 9" long handles. I naturally don't have access to an Aveo with the guts out to check this, but I THINK a pliars like that could be manuvered in some way in order for it to close onto the pump, with the handles in a place that you can get to. If it can, then back-and-forth circular/lateral movements should be able to break it free. If it's really on there good, then maybe the pliar handles could be extended with pipes to get more leverage. If doing that breaks the pump free, it won't hit the mating surface because it's still being held by the pliars.

Good luck and hope it works out well for you!