You can't turn off the "key lock" functionality (last checked on OS X 10.8.2). You can however install KeyRemap4MacBook, which allows you to make modifier keys sticky without this "key lock".
One caveat: There is a bug in KeyRemap4MacBook (last tested in version 8.0.33) that makes key sticking unreliable when typing fast. It may be fixed in future versions, but until then reliable sticky keys aren't available.