Can a tsunami be stopped by a head-on collision with another tsunami? With ordinary waves, the answer is "no", because a wave is not moving water, but moving energy, and two ordinary waves will not collide but briefly mix, then proceed on their way, much unchanged. To stop a wave, you need to absorb, divert or nullify the energy, which pretty much means walls, not waves. But if a tsunami is actually moving water, then perhaps a lot of the energy can be absorbed by a counter-tsunami, assuming you can create such a thing.
Mokalus of Borg
PS - From Wikipedia, it looks like a counter-tsunami would only be useful on land.
PPS - And that makes no sense.