ventilation don't stop on my imac

By "ventialation" do you mean the fans continue to run? If that is correct reset the SMC, this may take 2-3 attempts to work. To reset the SMC, shutdown the computer, unplug the power cord from both the wall and computer, wait 15-20 seconds and then restart computer and test.

I have the same problem, but on a macbook pro. What should I do? Thanks

See below.

Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support