The ban affects players who used Princess Peach in Super Smash Bros' "For Glory" online multiplayer mode, which forbids item use during matches. Due to a bug, however, Princess Peach is able to pluck turnips and throw them at opponents, circumventing the restriction. Nintendo subsequently banned a large number of Princess Peach players from using the game's online features after the bug was discovered, regardless of whether they used the cheat or not.
The bans are temporary, but many were handed out mistakenly, leading Nintendo to issue an apology afterward. Nintendo is currently working on a fix for the issue, and a patch should be ready by next week.