I don't believe that option exists. Assuming you have a valid reason for blocking a particular person, you could contact Etsy and explain and they could ban that customers account. That said, I haven't heard of them doing that, and you would need a very good reason with very good proof! As a business owner, you have the right to refuse business by canceling a transaction - I would put it in your policies though to protect you. Just remember if you cancel a transaction, the customer has the opportunity to leave negative feedback.