Requesting refunds can be the reason for some sellers to blacklist you, as refunds negatively impact statistics and some buyers will unscrupulously request refunds for many purchases.
To add a certain layer of protection for sellers is that each seller can set a certain threshold and if a buyer has been blacklisted by for example 3 sellers (although many sellers will automatically add buyers to the blacklist if they request a refund), then any other seller who has set a threshold of 3 will have automatically blocked this same buyer. Now, with a threshold of 4, 4 sellers will have had to blacklist a buyer, etc. This would explain why you aren't able to purchase from a specific seller that has not blocked you directly.
The same concept applies to a retail store when you see a sign on their door that says, "We Reserve The Right to Refuse Service, at any time for any reason". Therefore, there is nothing JVZoo can do to prevent you from being added to a blacklist. This is something that each seller determines on their own for their own protection.