Navision do not have month-end closing. In order to prevent users from posting records with older date, you can set the posting date ranges for a particular period. This can be set at the Geneleger Ledger Setup window. To open General Ledger Setup window, click FINANCIAL MANAGMENT --> GENERAL LEDGER --> SETUP --> GENERAL LEDGER SETUP
At the General tab, enter the date for Allow Posting From and Allow Posting to fields. If you do not enter any date in these 2 fields, users can post entries with any posting date.