Regroup gives you the ability to delete a scheduled post
From Manage Posts
You are able to cancel a scheduled post network wide by going to top navigation bar Posts > Manage Posts and choosing "Scheduled messages" from the left sidebar.
Once you have found the scheduled post you want to delete, choose delete from actions dropdown and confirm.
From Individual Scheduled Post page
Select scheduled post you want to delete, click on the title of the post.
Once on the scheduled post page, click the delete icon.