Just to let you know, removing a head from a body is actually really hard to do. It isn't an easy task even if you do have a blade.
You'd hacking and hacking especially if you are not a really strong character. And I mean really strong.
Before you go pointing out a movie like Kill Bill or something like that, one, that is a movie, two, the character tends to be quite strong, even if wiry and not buff and three, the weapon they have is never a cheaper version of the real thing. It tends to be a really good quality sword. The katanas you buy in game, are not as good as these katanas or blades. Lastly, the character is so highly skilled, they are legendary. Beyond your usual professionals.
You can, since this is a game though, if it is something your character would RP (hack their way through) then maybe with communication with a GM, this might be able to be done.
It just isn't an easy task. Easier than cutting through a femur, but still not an easy task. And messy as hell. You'd likely lose your lunch even if you are a killer.
Just some RP notes and possibilities there. :)