All species: Resettle
Maybe? Theoretically that would empty people out but I'm not sure if it would work.
Taxes at 100% would most likely cause the planet to rebel before the population was significantly depleted, and I don't think it would be necessary if your population policy is already "Resettle."
Unfortunately, there is a limit coded for population depletion such that the last 1M population is damn hard to get rid of, especially if it's your empire's native race. I don't think you can eliminate the last 1M of a planet's population short of bombarding it in any case.
If we're talking about your own native species, I'm not even sure how much the race-family population management policies really apply to it in the first place. I try to control population migration fairly strictly in my games and have the default policy on my planets set to "Not accept" for both my race family and all other and still my native race migrates to those worlds. As such, I'm not sure a "Resettle" or "Exterminate" policy would do much to them.
If we're talking about other races, you might be able to reduce them down to 1M then set an extermination policy to eliminate that last little bit, but remember that it's a blunt instrument. You won't suffer to big of a diplomacy hit for exterminating 1M population, but you can't be particularly selective about it. Your choices for that policy are either race family(several races) or all others(a lot more races). That said, this would only get you down to the last 1M on the planet, which you would then need to bombard as stated above.
Dumping a planet is currently just a huge pain in the neck unless you're OK bombarding your own worlds or with it going to someone else. I tend to use the editor to just erase the colony, then pretend I did all the rigmarole above. Saves time and micromanagement.