No, when you delete (disable) a user their information will not be lost. Their information stays preserved so you can still run invoices and reports on their time.
When running reports, next to your User filter you will see an "Include Disabled" checkbox which you can check and that will allow you to run reports on both Active & Disabled users in your account.
To disable a user you top right, click on your name and select Users. From there, click the user you no longer need active in your account and click the "Delete" button. Again this will not permanently delete the user from the account, it will disable them so they can no longer sign into the program and you will not be charged for them going forward. You can reactivate disabled users if needed.