Generally speaking, the easiest way to remove sponges is to scrape them off (the best recommendation I've seen for this is to scrape it off and suction it out).
Other suggestions include exposing the sponge to air (obviously not a guaranteed solution, and definitely not viable for this situation); hydrogen peroxide dipping the sponge (again, not viable here); injecting the sponge with hydrogen peroxide, vinegar, boiling water, or air; microbubbles in the display; and a few more. Predation is not usually a good solution for this issue