What is technically feasible in foreseeable future (a few ~100s of years) is nudging some long-period comets which already pass near Mars, into colliding with it. The necessary dV to adjust orbits is small if you do it far away from perihelion, say at something like Neptune's distance from Sun. To reduce damage to Mars, a very oblique entry into atmosphere may be best. Also, mining into the comet and putting in a several megaton fusion bomb(s) would allow you a controlled disintegration at a controlled altitude in Mars atmosphere. Another possibility worth looking into is aerocapture of the comet into a Mars orbit, and gradual dismantling (however I have doubts it can survive g forces intact).