Hi,

Confirm is normally made by the employees and as long as the attendance of theweek is not Authorized or project transactions approved you cannot stop the employee from updating.

If you mean blocking just updating of project hours there are some properties to set to define this.

Depending on what Time registration time base the employee has and the setting of properties in Company details you can prevent changes after Attendance Authorization of the week or Approval of the project transactions

Check the help text on these properties 

Regards,

Maria