How to Delete a WordPress User Account
To remove a user from your WordPress site, log in and navigate to the “Users” page. Hover over the user’s name that you want to remove, and press the red “Delete” link.
Important: if the user had created any pages or posts on the site, that content will be deleted along with the user unless you assign the content to another user.
If they have created content, you will be prompted with “What should be done with content owned by this user?”. From the select menu you can choose who to assign the content to, and then press the “Confirm Deletion” button.
If you do not want to delete the user, but do want to restrict their access to the wp-admin area of your site, you can instead change their user role. Navigate to the “Users” page and click on their name. Scroll down to the “Role” select box and choose the role you want to assign them. “Subscriber” is a safe role to prevent any backend access to your site’s content or settings.