Inject jQuery with something like this: http://www.brandonmartinez.com/2011/04/23/inject-jquery-onto-any-site/
Find the id of one of the <li>
tags for a file in the folder, e.g frx-list-item38220
Open the console and run this - it clicks the "X" button for every item in that folder.
$("#frx-list-item38220").parent().find(".removeFrx").click()
In Chrome developer console you can run: