Before Diamond & Pearl was released, Supporters and Stadiums were a subclass of trainers. After it was released, they became three separate classes.
Anything after DP that refers to Trainers refers only to basic trainers. Anything before then that refers to trainers will also include Supporters and Stadiums.
Spiritomb lies in the 'after' category, so it only blocks Trainers (not Supporters or Stadiums).