from: http://discussions.apple.com/thread.jspa?threadID=1369568

remark: I try in leopard 10.5.4

1. Create "calendar group"
2. Move many calendars into calendar group
3. "Export..." calendar from calendar group to .ics file
4. Create "new calendar"
5. "Import..." calendar file (from 3.) into new calendar (from 4.)
6. Delete original calendar group

Finish!