You guys need to think of this as personal security. If you think a situation is even the slightest bit odd, even if you have never encountered the situation, you should immediately question it.
I'm sure there are some players that played wow out there. How many times have people received emails stating that they were eligible for a free mount or a PTR trial. The email seems funny it comes from a name that looks like the host company. You click the link and the page looks just like the main login site. If you put your information and transmit it says something like your password is incorrect. Then it might have you verify your security questions. Once a person does that nothing happens. Well from that point said individual more than likely lost access to their account the security questions have been changed and all that fun stuff.
I know its a different situation from whats going on in game but the principle is the same. Someone is deceiving you to get something they want.
The Easiest way to prevent this guy from scamming anyone is to:
1. Limit who can invite to your guild.
2. Limit the access to your guild bank. (High value tab - limited access)
3. Limit who can promote characters in guild. (Make sure no one can promote to give access to "high value tab")
4. Verify from the main character that they are wanting to have an alt brought into a guild. (Don't take the word of a character that you have never seen before. How hard is it to switch to their main
5. For new members in guild, don't promote them until they can be trusted or prove that they are there to help the guild.
Everything I stated is minimal security. If you want to go farther that's great.
On another note. If someone stole your identity. Took credit cards out in your name, got a few loans, subscribed to new services, et cetera. It doesn't matter how that individual got your information, whether he impersonated you, rummaged your trash or hacked your computer, YOU are still responsible for whatever damage they did. Now there is a long and lengthy process of recovering your identity but most companies will still hold you responsible.
So please do yourself a favor and protect yourself and always question a situation that seems a little odd.