5th Gen Question About EV Training

So you beat up certain pokemon to upgrade your preferred stat.
Whats to stop random encounters from messing up your EV's? do they ever lock at a certain point? Do they stop changing after you use them all up?

Well, if you run from the fight, the Pokémon won't increase it's EVs. The other Pokémon has to faint in order to increase your EVs. If your Pokémon is too low level and it probably won't let you escape, just switch to a stronger Pokémon and run with him.
As for the "lock", if you reach 255 in a certain stat or if you reach 510 in total, you can't increase your EVs anymore.

Actually, yes. I think it's a maximum of 255 EVs or something to that effect for a stat, and you cannot max out every single one. So after enough battles, EVs are, as you say, "locked in". However, you can feed them EV-reducing Berries and have them gain other EVs.

Actually it's a max of 252 per stat. But however you can only get a maximum of 510 maximum EV's for your Pokemon. Therefore by saying this you will only be able to max out two stats, for example on my Alakazam. I had 252 special attack EVs, 252 speed EVs, and then 8 HP EVs. And once it hits that point it's locked unless you give it items to decrease the certain stat. Giving your Pokemon these items also helps if you mess up during EV training, gives you a second or third chance. If you have any other questions just let me know.

You can have a Maximum of 255 EVs in one stat.

Each Pokemon can have a Maximum of 510.

If you fight random encounters you get the EV of the Pokemon you defeat, so you'll get a fairly random assortment of EVs. To gain an EV the opponent Pokemon has to faint and every Pokemon that gains EXP from that battle gains EVs.

Please use Quick Questions for these sort of questions.