Which item wasn't paid for? If it's anything before the 2 phones you sold, then it's too late to do anything. If it's either of the phones, as already mentioned, select to cancel the transaction with the option that the buyer didn't pay. It's different to the old unpaid item dispute now. I've not had to do an unpaid since the new system came in, but I believe it's instant, instead of having to wait another 4 days before you can close the dispute.
Selecting that option will give the buyer a strike on their account. If they do it again in the next 12 months, they will find they can't buy from sellers who have blocks in place. Do you have blocks in place for serial non payers? If not, you should. It doesn't stop buyers from not paying, but it can certainly weed out the serial pests.
Edited to add: It's been around 15 years since buyers could get negative feedback. Buyers getting negs was completely pointless because it didn't stop them buying. A buyer could have 1,000 negs out of 1,110 transactions and they could still buy. Reason being, by the time they bought from you, it's too late to check their feedback and see they are a serial pest. Yes, if they bid early in an auction you could check feedback and maybe cancel the bid, but for the most part, it was pointless. Sellers getting negs can stop them selling. Buyers getting negs is nothing more than a short lived satisfaction from a disgruntled seller. It doesn't stop them buying.
