The options are
- opening the task for time entry
- assigning the resource to the tasks only when the task is started and not allowing unassigned tasks
- closing the timeperiods
If you need to keep 10 past periods open, resources assigned to tasks and task open for time entry and the resource does not enter time until the tenth week there is nothing you can do with those options.
An option could be a process which returns the timesheet when it is not for the current or previous timeperiod.