Unfortunately the Calendar widget comes as-is, and there is no setting for limiting which Calendar can be used. Calendar does a number of other processes to register the event when selecting a dropdown option. However, with some javascript/jQuery (and perhaps CSS), you could create a script that pre-selects a calendar and hides the dropdown.
The following small jQuery script is an example of a script that would select a calendar_id (ID 9 in this case), then hide the dropdown. Don’t forget to change the val()
value to the calendar_id you want selected.
<script>
$(document).ready(function () {
$('[name="calendar_calendar_id"]').val('9').change().hide();
$('div#calendar_calendars').next("div").show();
});
</script>
The hide() function will hide your dropdown after selecting a value.
Old Solspace Forums reference: http://www.solspace.com/forums/viewthread/12433/