Hello MayMay,
You will want to set your General Ledger Setup 'Allow Posting From' and 'Allow Posting To' dates to the current month, and as each month changes you will change these posting dates to the current month you are in to prevent users from posting into prior periods.
The User Setup posting dates are different. This allows a single user to override the General Ledger Setup. So this is useful for single users who are allowed to post into previous months. You can give a single user the ability to post for a certain date range (but without giving this to all users).