Comment 5 for bug 588539

A note for those who can't figure out how to kill the process: It's a python script, not the checkbox process, you want to kill.

-In System Monitor, processes tab, there should be a process running under the name "python". The icon should look like the "System Testing" icon.
-Right click and click "End Process" (then click End Process to confirm)
This will kill the hung status dialog and the checkbox process will subsequently end.

Or if you prefer the command line or can't find System Monitor, etc. you could run:
ps -aef |grep checkbox |grep python

to get the process ID (2nd column) and then:
sudo kill -3 <pid>

If that somehow doesn't work:
sudo kill -9 <pid>