how to restrict list in calendar event options area?

[The Ice Man]

Currently on our site, when somebody creates a new 'post event' in the calendar, there is a 'post in' area within the 'event options' section. the 'Post in' bit provides database generated dropdown list of ALL the bulletin boards in the forum, but I'd just like to restrict this list to 3 boards, presumably using the board's IDs. Can someone tell me how I would do this?