Anything that can be read by someone not logged into the site is also read and indexed for search by Google and other engines. This is the nature of the internet, we don't prevent it because we want information to be found when people search. That's the way most people find the site.
The only way to prevent it is to not post or to not to use your usual username(s). You can change your username and/or have your account and posts deleted, please send a PM to have it done. However, once google crawls, it holds copies of pages indefinitely in their cache.
