First of all let's figure out what is required to bring something down to Earth that is orbiting, say, 499 km up.
The lowest effort way to bring an object in a circular orbit into an orbit where it will intersect the Earth is the rocket firing at apoapsis required in a Hohmann transfer orbit. Thanks to wikipedia I know that the change in velocity required is:
where u is a constant that for the Earth is about 400,000 km**3*s**(-2), r1 is 6470 km (99 km high, just grazing the atmosphere. And r2 is 400 km more than that, or 6870. Plugging those numbers in you need to slow down by about 0.115 km/second, or about 415 km/h.
So if you line everything up correctly, your "gentle" impact has to change the velocity of the junk by several hundred km/h. Basically you're having the junk hit about as fast as a bullet hits, and are hoping that the junk bounces off in one piece and doesn't knock off any shrapnel. (Pieces of shrapnel would, of course, themselves become space junk that would need removal as well...) Also to have the collision happen at less than tens of thousands of km/hour, you have to set up a fairly precise rendezvous between the spacecraft and the junk, which maneuver will be the vast bulk of the overall cost.
The difficulty and risk of this maneuver doesn't strike me as remotely worthwhile. I believe it would make more sense to rendezvous fairly precisely with each piece of junk, take it into the spacecraft, and rendezvous with the next piece. The weight you save on the spacecraft (lighter = cheaper maneuvering) and lower risks more than make up for the extra bit of maneuvering you have to do.
EDIT: Clarified the shrapnel issue, and explained further why this makes no sense.
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