How do I clear the default email client?
The location you set your default email reader is in Mail.app:

Mail > Preferences > General

That will let you pick a different program than Outlook, but doesn't have a none option. 

There is a third party system preference that I have, but I can't tell if it is discontinued or the developer's site is just down. It lets you set a default application for all sorts of "various URL schemes, file extensions, file types, Uniform Type Identifiers (UTIs), and MIME types."
RCDefaultApp

One of the options under email is &lt;disabled&gt; which should be what you want.