DNN 5 introduces a new feature called "Soft delete", where a user is only marked to be deleted. Unfortunately, features like recover user and hard delete by an admin are still missing.

Here is a hack I did for a DNN5 project that needed users to be hard deleted. Alter DeleteUserPortal sproc:

ALTER PROCEDURE [dbo].[DeleteUserPortal]
    @UserID        int,
    @PortalID   int
AS
    IF @PortalID IS NULL
        BEGIN
            UPDATE dbo.Users
                SET
                    IsDeleted = 1
                WHERE  UserId = @UserID
        END
    ELSE
        BEGIN
            UPDATE dbo.UserPortals
                SET
                    IsDeleted = 1
                WHERE  UserId = @UserID
                    AND PortalId = @PortalID
        END

-- Custom modification to delete user records from db instead of soft delete

DECLARE @userName nvarchar(100)
DECLARE @aspnetUserId uniqueidentifier

SELECT @userName = username FROM dbo.Users WHERE UserId = @UserID
SELECT @aspnetUserId = UserId FROM dbo.aspnet_Users WHERE UserName = @userName

DELETE FROM Users WHERE UserId = @UserID
DELETE FROM aspnet_Membership WHERE UserId = @aspnetUserId
DELETE FROM aspnet_Users WHERE UserId = @aspnetUserId