I'm not sure what version of iCal you're using but here's how I can delete extra calendars:

1) Up towards the top left of the iCal page there's a button marked "Calendars;" when clicked a sidebar opens up which shows all of the calendars I currently have installed.

2) If I right-click on a calender it gives me options for that calendar and one of the options is Delete.

Try that and let us know how it goes.