Three ways. Click apple logo located at the top left of the menu bar and select force quit. Hold down the option, command, and escape buttons and select force quit. Hold down the power button untill the computer turns off.
What if none of these work? My MBP has been frozen for over 24 hours. I can't click anything and it won't turn off. I closed it and put it away for the night but the screen is still frozen. Please help.