How do you stop cooling fan that runs constantly?

Open Activity Monitor (Utilities folder).....click on the CPU tab.....from the top of that window's dropp down menu choose 'All Processes'.....look for a process that is using double or triple digit % of the CPU.....maybe something is stuck...

If you really want to stop it, you can just open the Mac, put a switch on the fan circuit, and close it.

Of course, this is if you have no use for the fan (like if it's in a cellar).

Thanks for the great solution. I too am finding my fan all of a sudden starts to run and won't stop. Did what you said and I find there are two Java applets running at over 100%. I know that if I put my laptop to sleep it will stop the fans and sometimes they won't come back on. Do you have any idea how to stop the Java applet from doing this? This would be the latest update to Java.

Perhaps a Widget in Dashboard?.....try canceling all the Widgets in Dashboard and add them back one at a time to see if there are a couple causing the problem.....if so see if there is an update too them.......just a guess😐