There are 2 main ways to do it. Buy planes and park them on the gates you don’t want to use. That changes the landing algorithm to reduce the number of arrivals accordingly. The downside to this is it clogs the cards at the bottom, I switched to using the top menu to find the aircraft that needed servicing and it was so much better.
Or there is the nuclear option and reset, you lose everything but your owned planes and destinations, but it’s well worth it, I did it when I had opened absolutely everything and got into the same situation. I had over 50 planes parked doing nothing, so one day I bit the bullet and reset, best thing I did, just remember to turn your cash into gold planes before you hit the big red button.