The switch to disable Cortana was removed, so now you have to use the group policy editor or (on Home), edit the registry.

http://www.zdnet.com/article/windows-10-tip-turn-off-cortana...

Also remember to block searchui.exe in your firewall or windows will contact Microsoft every time you run local filesystem searches.

You can't as of the Anniversary update [1]. You can kind of hide it though.

[1]: http://www.cnet.com/how-to/cortana-is-mandatory-in-the-windo...