Unless you've scheduled your campaign in, anytime!
A campaign can be deleted at any time from the campaigns screen. Select the campaigns you wish to delete on the left hand side.
Once selected this will change the top menu. This will now allow you to either copy or delete your selected campaigns.
However, the only time you are not allowed to delete your campaigns is if you have booked or scheduled it. This would interfere with your reporting and would make it difficult to find any potential required invoices.