How can you pick it up by mistake? It could be possible if you were to kill the katana-guy with a hammer, but that doesn't happen - it's a suicide.
Just don't pick it up.
woah you dont know how often i picked up katan by misstake in 1on1's, really many times. sometimes i even pick up 2 in a game (10 minutes). and well i have killed katana guy with hammer twice, once a long time ago and once just some minutes ago xD.

Just don't pick it up.
?