Everyone can make mistakes, and if you happen to schedule a post to the wrong social account or if you simply don't want it to be published anymore, you can delete it by clicking the X in the post options:
If you want to delete all the posts on a specific social account, just delete the social account from your Social Accounts settings and add it back on.
However, if you want to delete all of your scheduled posts in all of your social accounts, you have log into your Postcron account and then click on the following link: Delete All Posts
CAREFUL! When you click on the link all of your scheduled posts in all of your social accounts will be erased and there's no way to undo it.