But the main reason why Eve Online doesn't have gold farmers has to do with the game's structure. EVE Online is not forgiving unlike other MMOs. If you afk or bot in EVE chances are you'll end up dead, even in high sec. Though it's not the death you need to worry about, it's costs of death. When you're ship is destroyed, you will lose the value of the ship and the goods on your ship. This right here is the key to Eve's success on gold farmers. Gold farmers cannot run bot programs and simply go afk while their character farms for them. If they do, they'll receive kill mail indicting details on who and what destroyed their ship.
Ultimately, if gold farmers were to try farming ISK on their own, they will have a lot of barriers to break. They will need to spend weeks to months of skill training and they would need to spend many more active hours farming ISK. The time it takes for a gold farmer to farm about 750,000,000 ISK, the value of PLEX, during that same time they could have easily farmed tens of thousands of gold in World of Warcraft and made much more money. Simply put, it's not economical for gold farmers to farm ISK in EVE when there are so many other games that are cost efficient in producing and selling gold. Not to mention, to sell a character it takes a lot less time to level a character in WoW or other MMOs than getting a character to a decent amount of Skill Points in EVE.
So as you can see, it's not the PLEX system that deters gold farmers from being active in EVE. EVE deter gold farmers simply by its game-play structure. It's not feasible or economical for a gold farmer to try to sell in-game currency for real cash in EVE when they could make tons of more money in other MMOS that are much less forgiving on consequences and character growth. As a result this is how EVE won the battle against gold farmers.