Create a new contestant and play a solo game, getting as many questions wrong as you can and making sure to lose as much as you can in the Jack Attack. You'll end with a very large negative number as your average and this will make it excessively easy to get $30,000 over your average on your next play. Just don't lose too much, as it counts against your lifetime earnings and the related achievements.
If you do manage to destroy your lifetime earnings in this fashion, just move all your save data to an external memory unit or just delete it outright to gain a clean slate again.