[Plugin: Black Studio TinyMCE Widget] change visual/html tabs by user role?

By default the access to widget administration is enabled only for administrators, so users with less privileges cannot even access to the widget editor instances.
You may change these permissions using wordpress hooks or specific plugins, but the ability to use both Visual and HTML editing modes is a native WordPress feature, so it may be not so easy to get rid of it without hacking WordPress core code.

A quick and easy workaround may be to hide the Visual/HTML tab buttons via CSS:

– For Black Studio TinyMCE Widgets editor instances:

.editor_toggle_buttons {
  display: none;
}

– For Post/Pages editor instances:

.wp-editor-tools {
  display: none;
}