Open the Preferences using the button in the Enviroment section on the Home tab of the Toolstrip. In the MATLAB section of the preferences expand Keyboard and click on the Shortcuts sub-item. Search for action name "Comment" to see the shortcut that is currently enabled to perform that action. If Ctrl-R is not listed, I'm guessing you're using either Emacs or Mac shortcuts. You could customize that one action's shortcut or change the whole set to "Windows Default Set".