Cannot update checkbox with services
To un-check a check box with Services in Drupal 7 (aka setting false on a boolean field), try this:

"field_character_type": {"und": null}


I've just verified this works on my localhost (for both users and nodes), here's the payload for the PUT:

{
  uid: 1,
  name: "admin",
  field_character_type: { und: null }
}


And here's the path:

api/user/1.json

Make sure you include the [my-endpoint-path]/user/[uid].json for the call.

You only need to cast the data payload into a string if you're using vanilla .js and an xhr object, otherwise jQuery's ajax does it automatically for you.