This userscript applies to https://mail.protonmail.com
(after the recent layout update there's no beta subdomain anymore (beta.protonmail.com ) --- enabling 'beta' features is just a toggle button now (Settings | 'Beta Access' ).
It automatically removes the ProtonMail signature from the 'New message' textboxes,
which is appended by default to each mail body and cannot be modified via Settings in free accounts.

It uses the excellent library arrive.js

The script only works in Chrome, see here.
The script works and is tested on Tampermonkey and Violentmonkey.
Greasemonkey -any version- is not supported (the script might work, but is not tested).

If you have any bugs, bug fixes or ideas, please report at my GitHub repo