To delete your planned posts, first go to Settings > Default and add each social account to your default.
You'll do this by clicking the + and choosing any page, group or profile that has existing content.
Next, go to the Post section. There you'll see all social profiles, including their planned posts.
Hover over any of the posts to see a checkbox next to it on the right. Check that box and then you'll see a checkbox up top next to Planned, check that box to select all the planned posts.
Now click on the down arrow next to the checkbox to select the "delete all" option from the dropdown.