When you delete a user, their account will continue to exist for three days in the Deleted User Report found on the Insights page of your administrator account.
Sometimes you really just need a user gone forever NOW!
If that's the case, go to that Deleted User Report found on the left side of your Insights page. You should see something like this.
Press that red Force Delete button but be warned, if you go through with this, it is non-reversible!
Just to double check, you'll be prompted to confirm the action. If you press yes, they'll be gone for good.