There's no way you can force a buyer to pay for something, but there's a couple of ways you can deal with this, and the most appropriate will depend on a couple of things.
The first way is to cancel the transaction - you should have this in the 'more actions' dropdown menu, and if you click to go through that option, be sure to select the "buyer changed mind...." option, because it is at their request and the 'out of stock' option will result in you getting an unwarranted defect on your account.
The buyer is usually asked to confirm they didn't pay (or received a refund), if they confirm straight away, the FVF is credited, if they don't confirm within 10 days, the cancellation finalises and the FVF is credited.
The other way is to wait 4 days and open an unpaid item dispute, which you then have to wait a further 4 full days before you can close it and get a credit of the FVF. The UPI dispute is often recommended for frivolous cancellation requests because if it closes without payment, the buyer gets a strike on their account and the feedback option is blocked (or if they leave a neutral or negative beforehand, it will either be automatically removed, or you can ask eBay to remove it manually).
If I intend to open a UPI after a buyer has requested cancellation (I usually cancel, it just depends on how the buyer has gone about the whole transaction as to how appealing preventing them from leaving FB is), I reply and tell them that I'll initiate the cancellation process, which will finalise by X date (whatever date I can close it is), and that they won't need to do anything further since they have already indicated they no longer wish to purchase the item (the will get an email from eBay telling them of the potential consequences for non-payment of items, though - some buyers seem to take this as a threat from the seller).
