Thanks for the replies everyone.
After much mucking around with it I did discover that anyone could join or rejoin regardless of whether they were a member/registered previously or not. I was not expecting that anyone could just register to my site automatically. I did find the "Approve Members" setting thanks X-Ployt that seemed to be the only way around it.
James - does this ex-member rejoining bypass the application approval system (if enabled)? If yes - that's a fairly serious flaw.
I would like to know this also 4CAD, I plan to do some more experimenting to find out for sure.
To James/support the way I would have thought it worked would be that new people registering, can register through the site but that only gets them a standard iClan registration. They then must apply to join any other individual site in the iClan network using their iClan registration details. Does that make sense?
That's how it is done at other clan/guild website builders. Any consideration to making a change or adjustment to make this possible?