There are several ways to do this, but the easiest is just to run flyctl machines destroy on the machines you don’t need. The destroy command will handle removing the machines from the cluster before they’re destroyed.
Incidentally, for all reading, not just Brett, he says he’s just using PG for dev or testing and not for production workloads, which is a fine reason to use a single-node PG. But many users use single-machine Fly PG for production! Please don’t do this – it’s not a supported configuration! I’ve had to break the news to several users recently that their data was lost/may be lost when a host goes offline for a (suspected) hardware failure, taking the data with it. Always run a Fly PG cluster with 3 or more nodes in production.