Reset locked game software when colored "wheel" is spinning

Hello, Every now and then when playing Cribbage on my iMac, I inadvertantly press an inappropriate key. Sometimes this causes the little colored "wheel" to start spinning as if the computer is processing the input. The problem is that it never stopped spinning. Even worse, the computer cannot be shut down by normal means when this occurs. One literally has to unplug the computer, and then boot up again. Surely there is a better way (e.g., alt-del, or some other combination input). Does anyone happen to know what that way is? Thanks so much, Jack