Simplest way to delete win 8 is to insert the win 8 disk, select custum instal. On page showing your HDD, select advanced and delete the windows 8 Partition.
Bingo - gone. After deleting it, remove disk, reboot and power off.
Insert win 7 installation disk and re-install windows 7.
As far as 9 & 10. Simple, if you do not want, DON'T install.
PS make sure you back up any of YOUR files on your windows 8 HDD as once you delete the partition they are GONE-Baby-GONE.