Sadly, World of warcraft is heavily populated by "Those guys" as are every MMO, no matter how well you screen your chat, and groups, you will always encounter someone to spoil your gaming experience, i played MMo's for a long time until i got sick of hearing kids flaming, griefing and being generally disgusting to each other. THe problem with Online gaming, whether it be multiplayer xbox live races on forza 3, or hugely popular MMo's like world of warcraft, people ignore the EULA, forget that they are playing with other people, and think that because they paid for it, and because they are sat at their own computer and in no risk of being punched for being an obnoxious little turd, that their opinion is always right, that their behaviour is normal, and that they are the most important person playing and therefore anyone who disagrees with them is immediately a "N00b"
if you want to rule out any risk of having a game spoiled by someone else, you will have to skip online play altogether.