1) Create a new empty calendar from the iCal file menu, select which account you want to create it in.
2) If you have only a few appointments you can move them individually by editing them and changing the calendar in the edit options.
3) Alternatively you can copy all the appointments in one calendar to another and delete the original.
4) To copy a calendar ctrl-click on it from the menu that appears when you select the calendar button (top left corner of iCal) and choose export (export it to your desktop for ease)
5) Double click the exported file and choose the new empty calendar to import them into.
6) Ctrl-click on the original calendar from the menu that appears when you select the calendar button and choose delete.
I assume you are aware that if you do this, your calendars will no longer sync with any other devices.