Can't get rid of annoying Yosemite popup
When my computer boots up, I get a "Get OS X Yosemite free" box in the upper right corner. It has a button for "Details" but no way to get rid of the box. I right click, reboot, nothing works. It covers up icons and the search box. Any suggestions for getting rid of it, short of actually installing Yosemite?
MacBook Pro, OS X Mavericks (10.9.5)