It’s up to you to only allow trustworthy people into your faction. Not only that, if you were the faction owner and you didn’t promote them then they wouldn’t have been able to transfer faction structures over to private, only faction owners and admins can do that.
If someone had joined your faction solely for the purpose of stealing blueprints and then leaving that is against the rules.
If all they took was structures and stuff inside them then that’s not against the rules.
" 1.3 Intra-faction Gameplay: If you think you have a thief in your own faction, it is YOUR responsibility to protect / keep track of your property and assets. HWS CAN’T help with intra-faction gameplay (ie. internal faction politics), being that there are so many players."
It’s up to the server admins if they want to help you or not. I’m just pointing out the rules to you for future reference.