Customer won't stop purchasing and then raising disputes for non-delivery

[will_456]

I sell software using Paypal as the only payment method. When I receive the payment receipt I send the customer a key number to unlock their trial version of the software and make it fully functional. They are told clearly it may take 24 hours for delivery as I verify and test the key before delivery. This has worked well for many years.
A customer purchased and apparently didn't receive the key within 24 hours. This occasionally happens when their mailbox is full, the server rejects the email or it is lost in Junk or spam folders and I can normally rectify it quickly and get the key delivered promptly.
However this customer didn't notify me that they hadn't received the key and instead raised a dispute with Paypal the next day for not receiving the goods. Before I received the dispute they purchased again and immediately raised another dispute.
I refunded both payments a few days ago and just now they have purchased again which I promptly refunded as I do not want any more disputes.
How can I prevent this situation from re-occurring. Can you block someone from purchasing?

[sharpiemarker]

@will_456 

You can block/remove whoever is a contact on your Send & Request contact list so not really if this buyer is not listed. Perhaps talk to customer service about reporting the account as a harasser.

https://www.paypal.com/au/smarthelp/contact-us      

Scroll down to click “Message Us” or “Call Us” option on the bottom.

Or reach out to PayPal at Facebook or Twitter:    

PayPal on Facebook

PayPal on Twitter

[will_456]

I have more than 7000 purchases but my Paypal Contacts list only contains a few names (friends and family that I have sent and received money from).
So I cannot block this person.
There is no way to add him as a contact. I tried the option of syncing my phone contacts with Paypal which imports them all but even when his name and Paypal account email appears on the list there is no BLOCK option against it.
I was thinking of sending him a few cents which would add him as a contact. Is that a good idea? I'm getting desperate.
I've opened several chat windows with Paypal and as soon as I explain the steps I took that didn't work they stop replying and it times out.