Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the Name column header to display in descending order. Select the Safari process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that works. You may also need to kill any open tab processes that will be listed under the Safari entry.
This is from my computer. Note the several tab entries listed underneath Safari. You will need to kill each of those you find before you can kill Safari itself. Note you may have different items listed from your computer.