One buyer making all of my stock pending

I have one buyer that every time I put something into stock, they are buying it almost instantly but they are being held as pending for many days. I only know this as one transaction accidentally completed earlier which allowed me to see other orders by the buyer and I could see that there are 8 of the same item pending for them (but different to the one that completed). All of these items are selling at over £600 so it is going to have massive financial implications if I can’t sell them.

The order that completed has a business name on the delivery address that I looked up and it says the business description is selling online, so I am fairly sure they are a competitor trying to put me out of stock.

The delivery address that is given also has an email address half way down it, as well as the country France on a UK post code which I suspect is to give an undelivered item.

Is there anything I can do about this? I contacted seller support and they said to message the buyer to resolve it. I have done this questioning the delivery address, but they haven’t replied. There must be a better way to stop a malicious competitor putting all of my stock on hold?