The Joomla help button is controlled by the core of Joomla and unfortunately can't be turned off.

As a workaround, we can hide it via CSS:

Edit the template.css of your active admin template (ex: for AdminPraise 2 edit /administrator/templates/adminpraise2/css/template.css)

and add this bit of CSS to the very bottom:

#toolbar-help{display:none;}