• Log into Web TimeSheet with Administrative rights.
  • Click on Permissions on the left side pane.
  • Edit the permission assigned to the user.
  • On editing the permission, under System you will find Time Off expand this option by clicking on the '+' sign.
  • Uncheck the option Submit time off bookings.

Note:
You can even allow or restrict a user to Edit or Delete their future Time Off bookings by checking or unchecking the 'Edit/delete future time off bookings' option respectively.