If you take it all apart, the "axle" that the numbers ride on is melted into the gauge plastic. A soldering iron takes care of that. Then you get a drill with a big rubber bit (like you use to remove decals/pin stripes etc) and roll em back. Takes a steady hand and alot of patience Dont go too fast.. It is illegal to display the car as having less miles on it, but this isnt your point obviously. Get her back to zero, re-install, and melt it back into place with the soldering iron.
I did it one time just to see if I could. Took like 30 mins to get 50k off. long story short, i got the big drill and a bigger bit and went nuts on it, broke it to shit, and tossed it in the trash. I had just traded that cluster out for one with a tach so it was heading to the dump anyway.