Even really simple rule based bots like Turbo Texas Hold'em can beat most humans. Pokibot, developed by the university of Alberta was able to beat even somewhat experienced poker players at limit holdem using a few simple rules plus opponent modeling using action frequencies.
PokerStars has one of the best bot and collusion security prevention teams in the industry. They take this very seriously. But of course whenever there is money involved in anything in life, people will try and cheat. So it doesn't stop people from trying to run a bot on PokerStars.