To low level format a hard drive, the best is to clear it with all zeroes or some random value.
One of the easiest way is to use the cat command line instruction with the zero device:
cat /dev/zero >/dev/sdb
The zero device is faster than the random device. Also we clear the entire device (/dev/sdb) and not just a partition (i.e. /dev/sdb1).
Although some people will be able to restore the data, even after you cleared it this way, they'll need some $100,000 gear to be able to do it. They better need your data really bad (as in: make at least $200,000 in return, although they may