Can I stop a shutdown in progress?

Every once in a while I get prompted for a software update from a program that I use and I download it while I'm working on other things. Usually, once it's done it tells me that I need to reboot for the update to take effect. Since I don't want to shut down at that moment, I tell it "No, Thanks" where that option is offered, or I just close the window using the "x" in the corner of the window. That usually works.

However, every once in a while this doesn't work and everything starts to shut down against my wishes, even in some cases where "no" appears to be a choice. Long story short, Is there a way to stop that once it starts?
Thanks.