[imnuts]

You can't "unflash" the ROM. Once you flash a ROM, the phone is running that ROM, and if you hard reset, it just goes back to whatever was with the ROM that is on the phone when you started the hard reset process. If you flash a custom ROM and don't like it, or an official update and don't like it, you can flash the stock ROM back to the phone. There are links to all of the official stock ROMs in the upgrade forum. Be aware that flashing a ROM wipes all data from the phone (but not the storage card) so be sure to sync your contacts, etc. before flashing.