You can turn off "Custom Order Requests" by going to your "Shop Settings", then "Options". You can then choose the radio button to disable the custom requests.
The "Custom Order Requests" have been working really well for me in both of my shops. I think it's brought in sales for me. My buyers know that they can ask for changes.
However, I can see where it might not work for some shops that are OOAK, or their items are already made.