I think everybody on the forums sympathises with you for having had a spy in your ranks, although it has happened to most of us.
However, this has drawn some positives, you have had this put on the forums and some people have not been kind to you about it, but some things that are generally considered mistakes can be acted upon, if you so wish.
May I suggest that you check a player on tw-plus before inviting him or her, and discussing with them briefly why they want to join the tribe, this can help breed familiarity and a more trusting and close relationship within the tribe. Finally, if a number of players from a tribe attempt to join you within a couple of days with no real reason then be suspicous, and stop inviting for a couple of days maybe.
I sympathise with what has happened as a spy or turncoat can be darned annoying, although learn from your mistakes and have a good game
Finno