The more I look at it, the more my world seems to resemble the world of Avatar the Last Airbender, but instead of benders I have casters which are capable of unique talents (combat magic, enhancements, healing, psychics, elemental control, energy control etc.).
At the current state of development, the world is effectively in the medieval age while one particular nation is well within the industrial age. While the rest of the world still relies on carriages and ships powered by sails for transport, the most advanced nation has access to railways and warships. Their latest invention so far is a giant ship that specializes in carrying vast numbers of smaller combat ships, basically a medieval ship carrier which I'm also working on and I received help via this question.
I need a reliable and feasible way to deny my fantasy world any sort of aviation and remove the invention of flight from the equation altogether, because I don't want it to take shape of our modern world where Aircraft carriers are the undisputed rulers of the seas and any sort of threat can be destroyed with a well placed bombing run. I don't want any sort of planes or any sort of flying contraptions in the air, but I'm having serious difficulties coming up with a rational explanation to prevent such an evolution. After all, birds and flying creatures have always been around and I have no idea how to prevent the people from looking at them without trying to copy their flight and eventually succeeding.
PS EDIT: Someone in the comments asked "How absolutely no-flight must answers be?" To be honest, at first I was considering an absolute handwave of everything even remotely aerial, but given all the responses I'm genuinely confused where to draw the line. Ballooning? Kiting? After a bit of discussing, seems the restrictions would be drawn at any flight contraptions that move at a considerable speed similar to airplanes. Balloons and kites aren't of much use if you have strong casters within the enemy lines who could firebolt them out of the sky or even stronger casters who could manipulate the winds to screw them over.