So... it only applies to the Weirdboy, because his teleport is an attack instead of a move. If the Stormboy or Mek try to fly/teleport away they'll still have to make a flee role.

So, I just looked up the Weirdboy rules in WH40K, and the Weirdboy's teleport counts as a move. So if you Da Jump the Mek, the Mek can't then move that turn... but conversely, that also works for the enemy. You could grab them and plonk them down in front of your friends and they can't move themselves.