The damage percentage is a bit misleading, as they'll be destroyed as soon as they catch fire which seems to happen most of the time if you fire flaming arrows at it... that's the most you can do about it, really.
Never try to reason the prejudice out of a man. It was not reasoned into him, and cannot be reasoned out. - Sydney Smith
I've never tried it, but you might be able to use your own siege weaponry (scorpion, onager, etc.) to destroy them.
I was thinking this, but you'd have to sally out to do it most likely. A couple ballistae would probably work marvelously, but they'd get massacred outside the walls. You're inside walls, why give up those powerful defenses to fight outside them?