How can I stop Quickbooks from guessing which category to add bank transactions to?

Hi there, @janeascroft. 

Thank you for posting to the Community. 

It seems that you have created Bank Rules in the QuickBooks. Bank rules categorize all the transactions which you have set up.

To delete it here's how: 

1. Go to Banking tab. 
2. Click Rules. 
3. Look for the rule and under the Actions column, click the down arrow beside Edit, then Delete.
4. Hit Yes. 

Once finished, it won't be able to generate any transactions to which category it fall.  

For future reference, you may check this article:  

•  Download, match, and categorise your bank transactions in QuickBooks Online 

Please come back to the Community if you need further help. I'll be here to lend a hand. Have a great day!