Jessica,
You can use the import/export tool, filter using the campaign number and change the status to 'cancelled'. I don't think you'd want to delete alll of the activities, for historical puposes. If you want to delete them, you'd have to do that through customization. For more information on the import/export tool, check out the Help Center acticle here: http://www.methodintegration.com/method/kb.aspx?folder=methodcrm&article=Import-%2f-Export&id=189
If the campaign is set to be due on a specific date, the activities will still show, as the filter by view is set to select all or all before or on the current day. You could set a custom filter by view to filter them out. To do that you will need to edit the activity grid on the screen, and set it to filter out the activities you don't want. Perhaps something like future activities, as long as the current date is before the due date.
- Adam