> they have a right to have their opinion that the earth is flat
"Is the Earth flat?" is not the kind of question you can really have an opinion on. It has a definite correct answer ("no") which has been established by numerous independent methods by numerous people acting independently repeatedly over thousands of years.
For that answer to be wrong and the truth of Earth's flatness to have been kept suppressed for so long would require a vast conspiracy operating over centuries or millennia involving almost all political leaders, educators, and scientists, and a near total control on information flow.
That level of conspiracy could almost certainly only be carried out in some kind of Matrix-like simulation, but if we are in one of those then the question of whether or not the Earth is flat is superseded by the question of whether the Earth even exists.
What word(s) would you suggest to describe such people? The OED defines "stupid" as "Having or showing a great lack of intelligence or common sense", which applies here so "stupid" seems accurate. It defines "moron" as "a stupid person", so "moron" seems applicable.
It defines "fool" as "A person who acts unwisely or imprudently; a silly person". "unwise" is "not wise or sensible; foolish".
Putting that all together, "moronic fools" may be impolite, but it is accurate.