You can define a period where all the timesheets from the previous week or month will lock. This will then prevent your teammates from tracking time to them.
To set the Lock time entries feature, go to Settings > Time Tracking.
Here, you can set the schedule when the timesheet will lock. And you can choose if the timesheet will lock after a single week or a month.
After setting up the Lock time entry time, you can see the exact date when the next timesheet lock is set.
Selecting Send an email notification will remind your teammates that today is their last chance to add the time entries.
When the deadline reaches, the banner will pop up and notify your teammates that Timesheets will get locked today at 23:59. It will also show how many hours they tracked in the previous week/month, depending on the time locking setting.
By clicking on the X button to the very right they can dismiss the banner.
When the deadline expires, the timesheet will look like this:
Teammates who were unable to track time can ask you to enable it for them. Managers or Admins can unlock time tracking for any person.
To do that, go to Contacts > People and their profile, navigate to the very right and click on the sidebar:
... and you can select for how long will their timesheet stay unlocked:
Remember, this needs to be done for each person individually, not in bulk.
Also, keep in mind that for Managers and Admins locking time entries doesn't apply. They can unlock time tracking for themselves.