I recently switched from Nokia Lumia 920 to Lumia 950 (Windows 10 Mobile), and I can't find the option to disable default email signature in Outlook Mail app ("Sent from Outlook Mail for Windows 10 Phone"). I had that option on my old Lumia (Windows Phone 8.1).
Update : To reflect the UI changes after November update.
- Tap three dots at the bottom.
- Click on Settings -> Signature.
- Turn off Use an Email signature option.
If you want to disable Email signature for all accounts click on the Apply to all accounts checkbox.
Tap the three dots at the bottom, then go to Settings > Options and turn off "Use an email signature." Note that if you have multiple accounts set up you will need to do this for each account by using the dropdown at the top of the "Options" page.