How do I erase an Xcode 10 simulator clone through CLI?

Command to erase all testing simulators (you'll need to restart Xcode after that):

xcrun simctl --set testing delete all

Overall, it should be better to just reset those sims (you won't need to restart after that):

xcrun simctl --set testing shutdown all
xcrun simctl --set testing erase all

To erase specific one you first need to get his ID with list devices and then shutdown and erase:

xcrun simctl --set testing list devices
xcrun simctl --set testing shutdown 2BC2B50E-C4BA-45B9-9C73-AF5097BA1F0B
xcrun simctl --set testing erase 2BC2B50E-C4BA-45B9-9C73-AF5097BA1F0B

Thanks Scott McCoy for his answer.

I have not been able to find any interface with cloned simulators via simctl. You can completely remove them by deleting their folder under ~/Library/Developer/XCTestDevices. They each get their own UUID, but again, they don't get listed in simctl as far as I can tell.

You can list clones like this: xcrun simctl --set testing list

But any time I try to use device IDs with xcrun commands they fail with Invalid device: <device id>