I'm not sure how this would work in practice, but a gear puller might help.

Ernie's temperature shock approach might work, if you could cool only the pump. Dry ice might be useful, but I would avoid shocking the block with dry ice.

Is there a gasket between the pump and the block? If so, you might be able to put the assembly under tension and add some solvent at the edges, breaking down the gasket. This would probably not be an instant technique. You would need constant tension and renewal of the solvent periodically.