I think a robots.txt option such as:

Code
User-agent: *
Disallow; /forums/ubbthreads.php?ubb=calendar
Disallow: /forums/ubbthreads.php/ubb/calendar
Disallow: /forums/ubbthreads.php?ubb=showday
Disallow: /forums/ubbthreads.php/ubb/showday

Should stop all bots from accessing you calendar

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