You can target an individual by injecting that very individual with something lethal.

If that's not what you want, you'd need something like a virus to spread it. But then you have to ask yourself: what if that virus mutates? The specialization to certain gene markers is an evolutionary disadvantage, so evolution will tend to make it lose that restriction. Ooops.

I can think of ways to deal with that. I think the threat is serious enough that I won't post about those ways publicly.