The Year/Period fields are controlled by the "Allow Current Period Only" option in Group security.

In Group > Gen. Ledger, if the "Allow Current Period Only" option is set to Yes, the users that belong to the Group can only post to the default period.  If they need to be able to change the Year/Period this option needs to be set to No. 

Periods can also be password protected to prevent users from posting to selected periods.  To password protect a period, in GL choose Setup > General Ledger Profile > Open.  Enter a password next to the desired year and period.  When this period is going to be used in data entry the user will need to specify the valid password.