Stagefright isn't a virus, it was a vulnerability that could be exploited if the device wasn't patched. If the device wasn't patched, no number of factory resets will "remove" it. The best you can do is to take steps to reduce your risk of exposure to the risk.
Stagefright is the way in which Android displays media. someone found a hole in it that, when exploited, would allow them to gain access to the device.
Posted via the Android Central App