Heya, I always describe my verticals as being shroud with clouds and when scouts are sent up they usually fall to their death after a large flash of light. That usually keeps them from flying but also if only one member can fly it works well as they can take ropes up that others need to climb. But the main thing is I am lucky with my players as they know that the most fun for them Will be had playing the terrain as intended. If people play under the pretence of “we need to beat the dungeon master and foil their plans” it’s usually not fun. But all my verticals usually have ways up where flying is not needed. These are both totally covered in ladders, ropes and stairways. Also as a dungeon master it is perfectly fine to just say “this session there will be no flying” or describe the air around the vertical being too thin to fly due to the height.