Note: administrators can always delete users. Managers can only delete users if the account settings allow it.
Step-by-step:
Go to People > Users and find the person you want to remove.
Click on the name or the email address to open the user profile.
Click on the trash icon.
Save!
What happens when I delete a user?
When deleting a user, everything connected to this user profile will disappear. This means that even if you would create a new user with the same name and email address, the user would not be able to access their responses from previous surveys and would be asked to create a new password.