There's an easy way to definitely put an end to this debate.

An experiment must be done with a telescope during a perfectly clear day, starting when the sun rises on the eastern side of America.

A fairly big ship would travel in a straight line from the eastern most point in Canada, towards Europe.

The telescope would stand from a decent height, say 150 feet above see level.

If the telescope can't catch a glimpse of the ship went it reaches around 500 miles out, then the flat earth enthusiasts will have to concede.

On the other hand if a visual is possible as far out as 500 miles, then the planet being regarded as a globe would make no sense anymore.