OK, let's try the command line tool diskpart. Since it looks from the screenshot you're running 8.1, right-click on the start button and bring up an administrator command prompt from the menu.
type "diskpart" to start
"list disk" to view
"select disk X" where x is the one you want to edit
"list partition"
"select partition X"
"delete partition" use "delete partition override" if there's an error
if there's still an error, post it here
once all partitions on the drive gone, exit with "exit" and go back to Disk Management, where you should be able to create one big new partition from the clear space