Welcome to the Community, @geoffandros.
Cancelling or deleting bank rules in QuickBooks Online (QBO) is pretty quick and easy. Please refer to these steps:
Once done, you can now proceed with undoing the changes in the previous reconciliation. For more information, please refer to these articles:
In case you want to set and use bank rules again, consider checking out this article for further guidance: How to set and use banking rules for downloaded transactions.
Please know that I'm only a post away if you have any other issues or concerns. I'll be more than willing to assist. Enjoy the rest of the weekend!