Use EasyBCD [get it from neosmart.net/EasyBCD ] to delete Windows 7 from your MBR and then, format the drive that contains it.
Windows doesn't fiddle with your MBR at all unless you tell it to (through DiskPart or the Disk Management MMC) - it's not some Linux distribution that thinks it's entitled to trample over every sector of the drive. I know, but I never miss a chance to put the boot into Linux.