This is indeed a difficult problem to resolve. At the moment, the calendar installed is Webcal - but its not integrated with the forum at all. Therefore, someone has the tedious task of vertifying every new event listing before it goes live. Mass volumes of Spam listings are a killer. That issue would be perpetuated with any other 3rd party calender system.
A calendar that is integrated with the phpBB3 Forum itself would be the best solution - that way, only registered members would be able to create new events, and moderators should be easily able to manage / validate new items.
The problem is, there does not seem to be a reliable Event Calendar available for the phpBB3 forum software - all the one's i've seen referred to are either "beta" versions, riddled with complaints from users, or no longer available / supported...
The
http://www.meetup.com/Thailand-Motorcyc ... /calendar/ link, whilst interesting, does not solve our problem because whilsts its nice - its still a "3rd party application" that could not be integrated into our forum software. If we were using vBulletin forum software, it comes complete with an integrated calendar system.