First create a custom.css file for the ISIS template, then in this file paste the following:
div#toolbar-help.btn-wrapper {
display:none;
}
Clear caches and it should be hidden.
Update
To sum up the most important stuff of the long comments discussion:
@mrbuckmccoy: Make sure the custom.css is loaded with your browser.
Are you able to find the custom.css and your rules in your browser?
Try adding this in your custom.css body {background:#000;}
to see if you get any change -this should actually turn your background to black. Also remember to clear your caches and make sure you don't have other caching mechanisms that could affect the situation.
@mrbuckmccoy First confirm that we are talking for the latest Joomla version, which is 3.5.1. Custom.css won't work for versions before 3.4 or so. Then if you can confirm that this custom.css actually is being created and saved in your administrator/templates/isis/css folder.
If it exists there, and you have J3.5 then it should be loaded. If you previously added your css in the template's original css file and also didn't see the change, then it's possible that there is other caching somewhere else. It can be from htaccess, from the server/hosting, or 3rd party services e.g. cloudflare.