We installed a third party bundle with a custom record attached to transactions as custom sublists. We are planning to use it anymore but we don't want to delete the custom records as we have relevant data in it. We just want to hide the related sublist in transactions. Is that possible considering that the custom record and the related custom field are locked?

Some userEvent script might be creating that sublist on beforeLoad. You can just inactivate that and the sublist will not show up anymore.

I don't think it's a UE script. It's a native feature of NS. But even if it's a UE script, the scripts are locked

You can't edit or view the script, depending upon the permission. But you can always inactivate the scripted records even of locked scripts. But idk how that sublist is getting populated, you will have to figure that out.