The straight forward way is to just put a manual discount code into the pickup description, or on the product page near the buy button so the customer enters it during checkout. Though this may be abused by non pickup orders so you'd want to have che...

Hi there! I'm sorry to hear this.  This app is not mine however it looks like it may be helpful in your situation. As far as I know shopify does not have any built in tools to prevent this. https://apps.shopify.com/cozy-antitheft-for-images-and-more