To remove a user, you will need to visit the Users page in Company Settings.
Settings> Company Settings> Users> Add A User> Select User to Delete> Actions> Delete User
Warning: Remember to share the contract with appropriate users and / or administrators before deleting any account. The Administrator Access setting can also be activated to ensure that you have an overview of all documents on the account (under Company Settings > Preferences).
Please note: Removing users does not update the seats in your subscription. If you have a paid account, you will need to update seats from the seats management page in order for the changes to be reflected in the invoice.