Well these are some things that help you have a Pokémon with it's minimum Speed Stat, or will decrease it's Speed Stat:
Have 0 IV's in Speed.
Don't invest any EV's in Speed.
Have a Speed decreasing Nature (Brave, Relaxed, Quiet or Sassy).
Give it an item that reduces Speed such as a Power Item or an Iron Ball etc.
Let the opponent set up Sticky Web (not really advised unless your using Trick Room).
Hope I helped :)