Sometimes certain installed applications no longer respond and refused to leave. After using in vain the Exit option from the application menu, there remains the function Force Quit to get out.
The first access to this function is in the Apple menu -> Force Quit,
a window opens with the list of running applications. Just click on the application recalcitrant and click on Force Quit.
Now you know some of the best known techniques to quit an application recalcitrant.