To "nuke " the efi of a disk then nuke the first and commonly the last bits of the disk (/dev/sda in this example)
The first bit is the partition table so
dd bs=512 if=/dev/zero of=/dev/sda count=2048 (count=1 should suffice)
should be more than enough
if necessary the efi partition also (probably not)
dd bs=512 if=/dev/zero of=/dev/sda count=2048 seek=$((blockdev --getsz /dev/sda - 2048))
but without a partition table than gpart, gparted or gdisk would probably get you back to an MSDOS schema if the disk is less than 4TB., many BIOS's are broken with EFI booting, if you don't need it, don't use it.