Factory Reset isn't what you're looking for. You want the Android equivalent of reformatting your hard drive.
Since there's only one filesystem in Android's underlying Linux OS, that means that you need to unmount the memory chip and run something like the "mkfs" disk formatting utility on it. It might be possible to do something like "rm -rf /data", but unmounting ensures that no apps will be referencing the files when you pull the rug out from under them. Also, rm doesn't physically erase the data.
If you want to erase data from other areas than the removable memory chip(s), you'll have to do that on a case-by-case basis. Doing a "rm -rf /" probably won't work. And if it did, you'd probably end up with an unbootable device.
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