You can control whether your employees can add manual timesheet or not. To do that, go to Settings> Timesheet settings.

Now, you can turn off Manual Time using the toggle button. After you do that employees won't be able to add manual time.

Now, your employees won't see the option to add manual time anymore. So, they won't be able to add manual time by themselves.

From Timesheet Approval section, you can turn on/off the timesheet approval for any members. You can either do it for everyone at once or by individually. You can choose to turn it on/off for everyone and then customize too.