Currently, there isn't a built-in, one-click option in QuickBooks to adjust all customer ending balances to zero simultaneously. You'll need to make these adjustments individually for each customer. I have a tip on how you can do this in QuickBooks, jdrichardson.
You'll want to search for third-party applications that integrate with QuickBooks and provide bulk management features. This can facilitate bulk adjustments or mass transactions, making it easier to manage large numbers of customer balances. I've added a screenshot for your reference:

To know more details on how to fix the beginning balance once you start reconciliation, read this article: Fix Beginning Balance Issues in QuickBooks Desktop.
Additionally, QuickBooks offers a variety of customer reports that you can run to see your customers' balances due. Learn how to customize them to ensure you get the data you need. I've added this resource for your reference: Customize Reports in QuickBooks Desktop.
If there's anything else that you need help with, customers' ending balances, or adjusting their balances, let me know. I'll be more than willing to assist you.