Well this is nothing that I think yuo just can turn on (unless there is an update in the matter i've missed) so I would make a small plugin that in its init add a listener to the on paste event and there use the tiny cleanup code.

Here is a post about it: http://abeautifulsite.net/2008/10/forcing-paste-as-plain-text-in-tinymce/

And here is a link on some info of creating plugins: http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-6-RC1/Adding-a-TinyMCE-Plug-in/ 

A late response but no other one is present so here is my solution

https://episerverblog.wordpress.com/2015/04/24/override-tinymce-settings-for-paste/