In Pokemon diamond how can you git rid of HM moves?

Make your way to Canalve City. Upon entering, you should see a house north, and a second house north of THAT house (just beyond the PokeMart). Go into the second house where an old man lives. He will offer to delete any move you like, including HM moves.
If you fly there (and obviously landed at the front of the local Pokemon center) Go south to the nearest house.
Good luck!