Unless a forum is password protected posts always show on searches. I only know one writers forum that is password protected.
Personally I'm not sure what difference it makes people being able to see the posts on google or just signing for free and still seeing the same posts here. Also not sure why people would google to see post?
You can't copyright an idea so unless you post your whole book and someone copies and pastes it as it is there's nothing to steal. Posting an extract for comments/feedback is not the same as being published either.