How to block Ultarsurf through our Linksys router?

Am trying to block certain sites from our employees at the office. We are on adsl router


Linksys WAG120N, the problem is that they are bypassing and opening the blocked sites using ULTARSURF. Is there anyway to block this application, through the admin interface of the router?|||You will need to use more active filtering software such as Smoothwall on your server. There are others around that are equally as good. We use Impero, but that's really aimed at Schools