There are several ways of achieving this desired effect.

1) Find the place in ROM which stores the Experienced earned from each creature, and zero it out. The major drawback to this method is that it would take one Gameshark code per monster to accomplish.

2) Find the routine which adds Experience to the grand total after a successful battle, and rewrite it to add "zero". (This could likely be done with a handful of GS codes.)

3) Cut into a routine that occurs on a frequent basis, such as the frame counter. Add a special routine which zero's-out the Total Experience slot in RAM. (This is also unfeasible with GS codes.)

My recommendation is Option #2.