I'm saddened to hear that your business is closing. I wish you all the best for your future endeavours. I would be glad to assist you with closing your QuickBooks Online account, to prevent future billing!
Please be advised that you will continue to have read-only access to your company data, for one year from the date of cancellation. It's in your best interest to export your data and keep your information on file, should you need it in the future. Please refer to this guide, to learn how to download your data from QuickBooks Online. When you're ready, here's how to cancel your QuickBooks account;