Unable to shut down MacBook Pro

Dear Beloved MB Pro family, I inadvertantly started a System Diagnostic Tests in my Activity Monitor Screen and canceled the process a few seconds after the Diagnostic Test started. I now have the dreadful spinning wheel with the words "Canceling System Diagnostics" stuck on my screen. I am unable to force quit this task in my Activity Monitor since it is my Activity Monitor that needs to quit. I am unable to "shut down" my MB Pro because the Activity Monitor keeps interrupting the shut down. Any bright ideas?