The absolute simplest way is to start typing until the wrong one shows up in the choices that start with what you are typing, hit down until you get to the incorrect one and then just hit delete while it is still selected in the drop down window(don't click on it just scroll down to it and hit delete on the keyboard). That will remove the incorrect address from your suggested contacts list. If that doesn't work then you may have to wipe the whole file.
Follow this for step by step instructions on wiping the whole file but definately try the above option first.
http://support.microsoft.com/?kbid=287623