can repulse prevent "Harnessed Dragon" kill effect?

Yes — as long as the unit Harnessed Dragon targeted is at a battlefield, Repulse can counter the kill trigger. "When you play me, kill an enemy unit" is a triggered ability that goes on the chain and chooses exactly one unit, so from your side it's an enemy ability choosing a friendly unit and no other friendly unit — exactly what Repulse asks for. Repulse is a Reaction, so you can play it while the trigger is on the chain, and countering clears the ability so nothing dies (Rule 425.1.a).

The catch is Repulse's own target: it must choose a friendly unit at a battlefield. Harnessed Dragon has no location restriction and can kill a unit sitting in your Base (FAQ #5926), and in that case you can't legally play Repulse at all — the ability still resolves.

So: yes if the victim is at a battlefield, no if it's at Base.