According to the documentation there used to be a delete user button and also an option to delete in the user context menu:
Has this functionality been removed? Hopefully not, because some of my customers has the need for deleting users, and it will be to bad to plumb this in my self every time SF comes in a new version.
/Teddy
It is, as a standard, bad mojo to allow users to be deleted. In fact, Microsoft and other security tools and systems prefer making users inactive instead of deleting the user. If you'll notice, you can delete a user through the design-time editor, just not the run-time.
As I mentioned, if it is really, really important for your app, just build a dialog to facilitate this. If you are only using the user to grant access (and never store the user ID anywhere) then you only have to deal with removing the links to roles and permissions. We can help out if you go down this road.
And I hear Aruba is lovely this time of year