Attention World of Warcraft players!
If you do not want the internet to automatically discover who your alts are, please follow these step by step instructions.

• Hit ‘Escape’ or Click on the ‘Game Menu’ icon in your toolbar
• Select the ‘Interface’ option from the Game Menu
• Select the ‘Display’ category under the ‘Game’ tab
• Check the last box which reads “Display Only Character Achievements to Others”
• Rinse and Repeat for all your Characters

What this will do is restrict your account wide achievements from displaying when another player /inspects you ingame, or when they visit your character profile on the Battle.net Armory — but MOST importantly it hides that data from the API.  The API is the programming interface that other websites can use to harvest your profile data.  A few of them have started to parse through your account wide achievements to find out which alts earned the achievement, and then they are linking your alts together on their website.

WoWProgress posted about their Alt Tracking.  And WoWInsider reported on GuildOx’s addition of the feature.

For the time being it seems that if you were already refreshed on one of the tracking sites before you checked this option, your alts are permanently linked.  Hopefully the programmers of the popular sites will provide a way for people to dis-associate their toons, or even better, re-check during their refresh and if the information is no longer in the results of the API, remove the links to peoples alts.

Update Oct 26 It seems WoWProgress has added an option under your account settings that allows for “Hide my alts (you will need to confirm at least one character)”.  So if you go through the process of registering on their site, confirming one of your characters (they’ll ask you to un-equip certain items, logout and refresh) that are linked together, then checking this box your alts can be hidden.