There are several different Fantastic models, with slightly different controls, so yours may differ from mine. I can stop the dome motor at any point by turning the fan power switch (mounted on the face of the fan) to off, or I can manually open/close as much as I like using the black knob - pull it down and turn.
The dome will also stop moving if you simply hold it in place, but you would have to be up on the roof to do that!
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL