Difference between “Development certificate” and “Developer ID certificate”

Each year when you renew your program membership you can remove devices added in previous years. So if you had 100 devices allocated, you could delete 5 and add 5 new devices.

As soon as you add a new device, you can no longer remove devices for the remainder of that year.

So what they are telling you is each year, after renewal, delete any devices you need to remove first. Once you add your first new device, you are only permitted to add devices until your next renewal up to a maximum of 100 devices.

You can simply let the Safari membership term run out, as it doesn't look like Apple has given people an easy way to do this. Your best bet at cancelling it immediately though is to call Apple directly at 1-800-633-2152 and ask for them to cancel it.