We can complete a project we can still post invoices etc against it. Is there a way to prevent posting or at least warn if posting to a project with a completed status

Thanks for reaching out, accounts496.

Let me share some information about your concerns on the project with a complete status in QuickBooks Online.

Currently, having an option to prevent posting or at least warn you for posting to a completed project is unavailable. 

I recommend sending us feedback to improve the program and make changes to QBO to meet your business needs.

Here's how:

1. Select the Gear icon at the top, then Feedback.
2. Enter your comments or product suggestions. Then select Next to submit feedback.

Feel free to read this article for more details: Projects FAQ.

If you have more questions about projects or other concerns with QuickBooks Online, please let me know in your reply. I'll be around to help you some more. Thanks.