I had this issue myself with an old iMac and iPhone 4 I sold 2 years ago... I'd wiped them, and then they showed up after over a year - WTH?
There's only 2 ways to get devices I know are to, from the Find My iPhone app or the iCloud.com portal are to either erase the device or wait until the device is offline. Erasing the device gets you the option of removing the device once you get past the "erase" button you just clicked and I've seen the option to remove the device when it's offline (I tried this with one of my iPads, one of my iPhones, and one of my Macs) - clicking/tapping on the "remove" option gets you the warning you'd expect, in essence you'll never get to track it again.
I "erased" and "removed" the iPhone 4 a few weeks ago, and "removed" an offline iMac from my iCloud portal about 6 weeks ago - and they're gone! from my account after a year of looking at them. IMO Apple's made this "remove" feature really work for once! There's a guy out there with an iPhone he'll need to restore, but I don't have to look at it anymore.