Julie here from Shopify Support.
If you're seeing a Delivery Instructions box in your checkout, it sounds like you have local delivery set up for online orders. This can be done through either a local delivery app or directly within your Shopify admin. Are you wanting to simply remove the option for customers to include delivery instructions, or are you wanting to disable local delivery altogether?
If you want to remove local delivery altogether, and if you've set this up through Shopify, then you can remove this option by heading to Settings > Shipping and delivery > Local delivery > Manage. There, you'll just want to make sure that you uncheck the This location offers local delivery option for any location listed there. Once that's done, you should no longer see the Delivery Instructions box in your checkout and local delivery will no longer be an option for customers.
If you've set up local delivery using an app, however, then you'll just need to look into the app settings or uninstall the app. Keep in mind that some apps may require additional uninstallation steps before the app can be completely removed from your online store. For this reason, it's a good idea to review your apps' help documentation to confirm if this is the case. You can generally find help documentation for third-party apps directly within the app itself, under a Support or Help section.
If I've misunderstood your question, or if you are still experiencing issues removing this option, please reply here and include your store's URL. I'd be happy to take a look and offer some further guidance.
Looking forward to your response!