This kind of block will only happen when an individual makes multiple free checkouts in a day, there are no blocks for regular purchasing. This is a security measure to keep one individual from abusing a free offer or a misconfiguration in your discount settings.
To keep buyers from running into this block you might want to change how you make free offers, if there are multiple free products to purchase they should at least all be taken in one go rather than ordered individually. It may not even be necessary to involve our cart in free offers at all.
Also if any merchant ever runs into this block during their own free test purchases it can be cleared by logging into E-junkie from the IP address in question, although this won't work for a buyer who has hit the block, they will need to wait 24 hours for the block to expire.