How do you steal Pokemon from the battle factory?

You can't.

Get a gameshark sp preloaded with Pokemon emerald chaets and select the warp cheat and clik the battle frontier. Then pick a Pokemon you don't need and go to the factory. Select 3 Pokemon to steal and when he takes you to the room where you batlle as soon as the screen goes black press L

You can but you need action replay or game shark. I have action replay and this is how I've done it before.

1.deposit your good Pokemon have something like a lv.3 wurmple.
2.make sure you have a warp code I normally go for the southern island one: F0EC26B8 31EC5657
3.have your action replay OFF as you enter the factory.
4.the guy will ask you to save, say yes and enter the next room
5.choose your Pokemon and continue to do one battle
6.while in the battle switch your action replay ON
7.win the battle and you will arrive in the top right corner of southern island
8.switch your action replay OFF then head out to the boat
9.take the boat to a city
10.take advantage of lv.100 pokemon

Supersmashbro93

(p.s. Do not warp without doing the first battle otherwise you'll end up on top of trees or out of the screen and you'll have to restart your gba because you WILL BE STUCK!!!)

Hope I helped