I couldn't find a System Setting (though I did find one to set its URL: base_help_url). You could change it to your own help page.
The help button is a span with the ID: modx-abtn-help
So this in your CSS might work to hide it from everyone (including you):
span#modx-abtn-help {
display:hidden !important;
}
If that works, and you want to hide it from specific users, you'd have to write a snippet or plugin that injected that CSS for just those users.
I think something like this might do it in a snippet in the template:
if(! $modx->user->isMember('Administrator')) {
$css = '<style>span#modx-abtn-help {
display:hidden !important;
}</style>';
$modx->regClientCSS($css);
}
return '';
Let me know if it works.