A Wanderer is a Giftia who's programming has been corrupted after 81.920 hours of service, and so for that they lost their memories and personality and only act on instinct. After that a Giftia has turned into a Wanderer, it's almost impossible to calm them down and simply deactivate them, that's why often has been summoned the R. Security to kill them before they do any harm to any people.
Wanderers are mostly killed with SAI's CC (close combat) shock guns and R. Sec. M16's seen in the anime/manga,
Wanderers can't be rebooted, restarted nor reactivate, so that's one of the scariest things that can happen and why people say that having your loved one's turn into a Wanderer is the most terrifying thing can happen. After being changed into a Wanderer its impossible to recover it with a new OS so that's why they have to be eliminated with deadly combat weapons.