Load and execute this binary at 0x00080000 in RAM. It will force a complete blank of the NAND (even the bad block markers).

Other notes:

There is no way of telling if it has worked or even loaded sadly, so you just have to check PC and see where it has ended up. Leave about 10-20 seconds for it to work before halting and checking the register values.

http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,42,2674