If trample could be repelled at all, even by something like pikes, elephants would be much, much weaker. They have poor morale AND poor defense, and an elephant trying to trample a squad of pikemen would probably have to pass 3 repel checks. If en elephant does manage to get through, his friends might not be so lucky and he's all alone next to 20 or so pikemen.
Not that I wouldn't like to see units capable of repelling tramplers.
