It depends on the particular version one is running. If the popular Ubuntu has one, I don't know it. The one I am running (PCLinuxOS) uses ctrl+esc which brings up a list of running apps just like crtl+alt+del does in windows and one can then cancel what one wants. I did not try just closing the browser to see if that would stop it, I was so surprised when it popped up. Of course, in theory, one could also stop everything by just logging out, but again, I did not try any of those. As I mentioned above, I had never seen one like that before and my thought was that many users may just install an AV program and never try it on a "test virus" and thus not know what the AV software is supposed to do until too late.