I tested the plugin you linked with both Asteroid Theme and the default WP Theme (Twentyfourteen). Both theme’s display a “Calendar” title even without setting one. This means that the plugin is the one setting the title.

I inspected your calendar plugin and there is a certain line that prevents a user from setting an actual empty widget title.

$widget_title = empty($the_title) ? __('Calendar','calendar') : $the_title;

You could of course input a space ( ) instead of a title but that would still output the html markup for the title which would show an empty bar. This is out of any theme author’s control and is something you should ask the plugin author.

However, you could simply hide the calendar widget’s title through CSS:

.widget_events_calendar .widget-title {
    display: none;
}

The page still shows Calendar as a title. This in turn creates the calendar in another div that is not 100% of the sidebar

The calendar widget’s width is controlled by the plugin itself. As indicated on the plugin’s code:

$calendar_body .= '<div style="width:200px;">

Again, this is something you should ask the plugin’s author. Though you could also override it through CSS:

#events_calendar > div {
    width: 100%;
}

Note: As indicated on the notice on top of the forum page, Questions about specific plugins should be posted first on their respective support page as plugin authors should know more about them.

No problem. To override the plugin’s CSS, you can just add the CSS codes above on Asteroid Options -> Custom CSS