Deleting Users from Salesforce permanently

As of today, there's still no way to do that. You can, however vote on this idea, that's currently on review.

Those who thinks that this Delete feature is required for Deleting unwanted user details from Salesforce please click on below Link to Vote for the Idea. Currently this Idea is under "PRODUCT TEAM REVIEW". I believe this is a cool feature that all Admins required now or in near future.