Are requests for an emergency landing ever refused?
I can imagine a couple of reasons why ATC could tell the pilots to please land somewhere else even if they have an emergency


All runways are closed, e.g. they are repaving the runway surface or when the snow trucks haven't yet cleared a thick layer of snow or the instrument landing system is inop and the visual conditions are not good enough, runway flooded after hurricane,...
Another emergency in progress, e.g. passengers freely running around on the airport after an evacuation, not safe to land
Political reasons with hijacked plane (e.g. Lufthansa Flight 181 Landshut was refused to land even when they declared an emergency with low fuel) https://en.wikipedia.org/wiki/Lufthansa_Flight_181
Presidential no-fly zone when other airports are available close by.
Virus infected people on board to avoid contamination of those on the ground (zombie apocalypse)