This feature is available on Premium and Enterprise plans.
Simple! Just click on People, then on Users. Find the user that you want to block from logging in and click their Edit button. From there, scroll down and enable Disable Log-in.
When a user is disabled, they will not be able to log in. However, they will still retain all of their information and progress, and will be able to continue where they left off once their access has been restored. Their information will also be included in any reports that you run.