If you want to stop freelancing, you must quit your VAT ID (btw-ID). If you don't do this, the Tax Authorities (Belastingdienst in Dutch) still think your business is running and expect your turnover tax. To avoid getting into trouble with this, you should quit your business. Here the Tax Authorities explains exactly what you need to do. 

If you are not registered at the Chamber of Commerce (Kamer van Koophandel in Dutch), you need to contact the Tax Authorities to stop your business. (0800-0543). We recommend you always call the Tax Authorities first, so they can clearly explain the steps you need to take to cancel your VAT ID. 

Remember that you still need to file a final turnover tax when cancelling your VAT ID. Even when you have terminated your VAT ID, you are still obligated to save your records for the Tax Authorities for up to 7 years. So store your invoices and any other administration in a safe place.