You can remove exp. Here this is the trigger in my map that does so.
Trigger:
Hero - Add (0 - ((Hero level of (Triggering unit)) x 25)) experience to (Triggering unit), Show level-up graphics
Also bob666...
You can go lower... In my map I was level 1 and died and I lost exp and I had no exp and I went from 0 to -49087398723481
lolz but I gained some exp and the number went to normal.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.