There is currently no way to remove the sizing grip from the QMainWindow on Mac.
There is no Window flag to set the window to 'non-resizable', even when the QMainWindow has Fixed size.

Currently I am using the following hack to remove the status bar area with size grip, and then reduce window height accordingly.

void setupWindowWithoutStatus(QWidget* w){
NSView* view = (NSView*) w->winId();
NSWindow* win = [view window ];
NSRect fr = win.frame;
fr.size.height -= 22;
[win setFrame:fr display:YES];

NSUInteger mask = [win styleMask];
[win setStyleMask:(mask & ~NSResizableWindowMask)];

}