SHOUTBOX

How can a visitors post a message on my shoutbox, even if its visible only to members ? 

At first I was a bit confused since if the shoutbox is not visible to guests, how would they post.  However, maybe you are wanting a way for visitors to post to the shoutbox without using the shoutbox.

 Hello Geek_Girl, someone did post on my shoutbox without having the credentials....

It's a spammer as they would be the only ones that would have the knowledge or reason to do it.

It is possible to post directly to the shoutbox visible or not using a html form that sends a http post directly to your sites shoutbox url. http://www.yoursite.com/modules/?r=shoutbox/write_message/

Hidden from guests just means it's not shown to guests in the browser. It does not mean it's not available. Those that know what they are doing can still post to it.

can I prevent this from happening ? 

Not without modifications to the shoutbox code.

You may also be able to block the shoutbox url to guests in the Page Access Control module.

I will try, thanks....