Ignoring the ShareThis plugin on a footer widget

I have an odd issue with the ShareThis plugin.

Currently, it shows the list of sharing links in all the right places, except that it also includes them in the list of recent posts in the footer widget of my theme.

I don’t want them displayed in the footer list. It’s ugly there and pointless.

I could use CSS to hide them, by using something like

#footer .sharethis { display:hidden; }

and of course this hides the actual buttons but still takes up space because it makes room for them. I tried setting the height to zero but that didn’t work, the vertical space was still applied.

The ideal way to hide them is in the actual widget code. I tried the following:

if ( dynamic_sidebar( 'primary-widget-area' ) );
	//return empty string
            return $out;
	}else{
        //continue with the code

But this evaluates as false for all places where the plugin operates, including the bottom of posts where it should run.

How do I create my if statement to run only if the widget is operating in the footer area?