Go into the Expense, Check or Bill transaction that triggered the billable expense, and click the check mark (not "View") in the Billable column TWICE, to clear the box. Save.

To find them, go to Sales>Customers, and click the box that says UNBILLED ACTIVITY, click each customer and open the Billable Expense Charge. Then click the link on top left to the Expense, Check or Bill.

If there are many billable expenses, there is a utility to delete in batch:

https://quickbooks.intuit.com/community/Income-and-expenses/Remove-a-billable-expense-charge-from-th...

 I think I read that this does not work for QBD conversions. Let us know how it goes