You could throw rocks from Deimos have them fall on Mars. Hint: throw them in the direction Deimos came from.
Nope, would never work.
Or do you think that people on the I.S.S. could de-orbit simply by shoving off from the space station?
Throwing a rock as hard as you can from Deimos - in the retrograde direction - would merely place the rock into a slightly lower orbit (one with one apsis touching Deimos' orbit, and with the other apsis maybe 1 km lower).
HINT: It would take about as much energy to de-orbit from Deimos and impact on the surface of Mars as it would take to blast off from the surface of Mars and reach Deimos.
Regards,