Presumably the users doing this are in February and posting in March are sat there on 10/02 and are keying into the system that actually it is 12/03 - because as standard even if the delivery was scheduled for 12/03 and it arrived on 10/02 processing the receipt on this date processes it in Feb, so you HAVE to change it.
So question 1 - why are they doing this?
Question 2 - why are you allowing forward posting in the ledger - assuming you are setting creating a physical transaction on receipt?
The users make mistake when they post delivery note. It usually happen at month begining. Users does not finish posting purchase/sales order in Feb.
So in the begining of March, they try to post the delivery and invoice (Set the date as Feb). However, sometimes users forget to change the delivery date or invoice date, so the Date will be taken as March.
On Question 2:
Again it happen in the month begining. Since users have not finish Feb orders, so we cannot stop Feb period. So at month begining (1st week), we have open Feb (previous month) and March (current month).
Get the staff to book the receipts in in a timely manner - educate them on the issues they are causing.
Get the accounts staff NOT to process an invoice in until the receipt is processed. Accounts can alter the document date to reflect the invoice but process it with a financial date always after the receipt.