Copy this into your computer.dat file. You can now Reset Trainer Battles from any PC. It's not my code, it's actually from a topic from a few weeks ago. Anyway it only goes up to Azalea town, and doesn't add Gyms or Rival battles again. Davey had an excellent code that had options for those, but I never actually copied it. If you want to extend it to include new trainers it's not that hard to do.
EDIT: Found the thread, this particular code is either Davey's or an edited version of DracoHouston's, I can't actually remember.
Code:
@Text:Booting up the PC...*Choose a host to~connect to.
[USER=5463]options[/USER]:Bills PC,<playername>s PC,Hall of Fame,Reset Trainers,Log off
:when:Bills PC
@Text:Pokemon storage~system accessed.
@Action:StorageSystem
:when:Reset Trainers
@Text:You check your emails*Everyone wants a rematch!
@Unregister:trainer_30001
@Unregister:trainer_30002
@Unregister:trainer_30003
@Unregister:trainer_31001
@Unregister:trainer_route32\32001
@Unregister:trainer_route32\32002
@Unregister:trainer_route32\32003
@Unregister:trainer_route32\32004
@Unregister:trainer_route32\32005
@Unregister:trainer_route32\32006
@Unregister:trainer_route32\32007
@Unregister:trainer_route32\32008
@Unregister:trainer_route33\33001
@Unregister:trainer_unioncave\001
@Unregister:trainer_unioncave\002
@Unregister:trainer_unioncave\003
@Unregister:trainer_unioncave\004
@Unregister:trainer_unioncave\005
@Unregister:trainer_sprouttower\001
@Unregister:trainer_sprouttower\002
@Unregister:trainer_sprouttower\003
@Unregister:trainer_sprouttower\004
@Unregister:trainer_sprouttower\005
:when:Log off
@Text:Kolben OS~shutting down...
:end
:endwhen
:end