Two weeks is more than reasonable .
I would send the buyer one last message explaining politely if you don't hear within 24 hours a convenient time for collection, you'll deem the item to be unwanted, and will be cancelling the order which will fully refund them.
Then, if there still is no communication proceed this way:.
If you are on My eBay Selling, go right of the item sold and from More Actions select Cancel this Order selecting the reason.... Buyer Requests to Cancel. This refunds the buyer, refunds your seller fees, and your seller performance is not affected.
If you are on the Seller Hub, find the item in Orders >> Posted and Awaiting your Feedback. Tick on the little arrow left of the item from the drop down menu which appears scroll down a little, you will see Cancel this item, and complete as above.
Also block them from returning and wrecking any more of your sales here: