I try to do this:
I just want a widget to be displayed on 1 category page, then the rest of the pages shouldn't display anything (and this worked out fine, that's pretty simple).
But the rest of the category pages (where there are no widgets) should be in full width, and this isn't the case. They are still making space for the sidebar, even though there nothing there.
Since I'm not a coding wiz, I asked Justin Tadlock if was a problem with one of his themes that I'm using, but according to Justin:
It's because the plugin doesn't "deactivate" the widgets while on that page. They're still being read as active by WordPress.
I also posted this here.