I get it, It's easy to just blow it up.

But honestly, you could just make any colony you don't want into a sector of its own and vassalize them or give them to a vassal. If you really want to get rid of the planet, you could resettle all pops or even sell them on the galactic market.
And if you think "but as an egalitarian xenophile I couldn't do it" I would reply, a xenophile egalitarian wouldn't kill them all by blowing the planet up.

You could let them go as a vassal and grand that vassal independence to abandom them instead of genocide them.

[edit]One point i forgot, if you want to actually kill them all, you already can. It's the purging policy, and it takes a lot of effort to eradicate a people