If you need to remove a User from your account you will need Owner permissions, which is indicated by a yellow star in the User tab. The person who created the account will have this permission by default, but it can be transferred to another user.
To remove the User, click on the User menu (indicated by three vertical dots) and choose 'Remove'. You will be asked to type the word 'Delete' to confirm the action. Once complete the user will then no longer be able to login.
Note: No confirmation notification is sent to the User telling them that they have been removed. All of their actions within the App will remain visible in the Change History logs.