gcc hoge.c -s -Os -mwindows -nostdlib -lkernel32 -luser32

#include <windows.h>

void WinMainCRTStartup (){
HWND hBBCalendar = FindWindow("BBCalendar", "");
if (hBBCalendar){
if (IsWindowVisible(hBBCalendar))
ShowWindow(hBBCalendar, SW_HIDE);
else
ShowWindow(hBBCalendar, SW_SHOW);
}
}

どう？