Many people underestimate kinetic energy as an effective tool. If this comet were slammed with another comet of equal size going the opposite way at, say, 130,000 mph, there wouldn't be much left of either. Mostly just an ionized patch of gas, glowing at first and gradually fading out. It would probably create other problems in the solar system, especially with scientifc satellites and maybe Loral and other commsats.
Yeah, but... If you have the technical ability to grab another comet and enough energy to redirect its trajectory to exactly where you want it to be so that it hits the first comet head-on, then you don't *need* the second comet, you can use the same tools to just redirect the first one away from an Earth-impact trajectory... ;-)