You might be surprised to learn you can be booted off a flight for your attire, hygiene or odor.
That reason can’t be discriminatory, even on its face. And in some cases, like under ADA in US or EAA in EU, the requirements are even stricter. But yes, if the reason provided is not discriminatory or etherwise regulated by law, then they can refuse the service. But they can’t refuse for “any reason”.