For the Sims 3: If you want to delete a town from your game completely,and not just a game save, you have to delete it from the folder where it's located. It should be located in the InstalledWorlds folder, and the directory should look something like this: Documents/Electronic Arts/The Sims 3/InstalledWorlds Voilà! A word of caution: If you delete a town that is associated with any game saves you currently have, bad things will happen, so, before you delete a town, move your sims to another.
(Addenda, added by another user)
In addition, If you are talking custom worlds, then it is preferable for you to use the launcher: simply go to installed content button and find it and delete it from there. Removing it manually by going to the InstalledWorld folder on the Sims 3 folder in the My Document/Electronic Arts directory (as mentione above) also works, BUT it does not delete the saved game, which cannot be accessed. MILK FOR THE UNYUUFEX, FLAT CHEST FOR THE CUTENESS THRONE, SKULLS FOR THE SKULL PROBES (user talk:Mathetesalexandrou) 04:09, March 13, 2012 (UTC)