I couldn't destroy any ram, though in a couple of siege battles I used more than 8 units of yeoman, +3 experience, all trying to flame the thing. Now I go for direct attack against the unit that mans the ram and, if not enough, suicidal attacks just before the ram gets to the gate.

And if this fails, retreat to inner defenses and hope for mercy and/or luck .

Much more interesting, I'd say, sieges in MTW2.