Can anyone shed light on this for me or is it me being daft

Noticed the other day when the kids were playing under my profile that some total stranger had joined them, whilst I kept a close eye on them it did make me feel uncomfortable so I thought go through the options and only allow friends abit like the 360, couldn't find anything, the wife is really twitchy about this kind of stuff so i guess the question is how can I block everyone apart from friends? or can I make the game play in a "standalone mode"?

Thanks in advance

Chesser