The truth is that these 'members' are automated bots posting in a way that cannot be prevented, only deleted. Usually we can see the sign up, view the alarm bells and delete the 'member' before they can post. They don't appear as members until they have posted and it's all done in a split second - sign up, post, gone for good.
If I could prevent it I would of course, but I can only fight humans - so rest assured we will delete at the earliest opportunity.