I was targeting that specific element and could not override it, even with important. I spent an hours screwing around trying to figure out how to get around it.
You have to remember that the way you remove select dropdown buttons for IE, is the make the select element something like 120% wide and a containing element 100% wide with overflow x hidden.
That way you can do neato things with select menus like this:
Additionally, the end user can set those rules himself, and end user flexibility in this case is much better than reduced functionality due to inexperienced users.
I would not have been so annoyed with it if I could have reliably overridden it, but try as I might, I could not.
Another point is that when you are adding these styles, you are essentially adding inline styles, which is ugly and messy anyway. If you want to style them, then to do it correctly, you should really be calling a stylesheet and doing it that way instead of inserting them inline.
For me this is a ridiculously useful plugin, but at the moment I've basically had to locally fork it, which is less than ideal :-(