Roque,
Thank you for informing us of these opportunities.
I understand the problem with someone stealing a site name out from under you. Happens with company names, logos, internet domains, etc all the time.
I will offer a couple of suggestions based upon my involvement with various groups besides AWS on Linked In:
1) Use a slightly different name such as: 'American Welding Society Official Site'; OR 'American Welding Society Members'; OR 'American Welding Society, National'. I see these variations amongst other national organizations I am part of. People start unofficial groups with the organization name and even logo. Most are money seekers wanting you to then pay them to get your own name. Unless you have the entire name trademarked that is possibly the only way to get it. Look at some of the District and Sections for AWS such as mine: 'American Welding Society, Arizona Section'.
2) I see you at least got the logo removed from their usage. It will still be misleading. Probably my above suggestion is still a way to remove most of the confusion because members will realize that someone is golddigging by using our name. I was going to suggest that but then with a last minute check noticed it was removed.
3) I would suggest not paying them if that is indeed what they are after. Just go around them as suggested above. With the logo on our official group and a group name that may lead members down the correct path I think you can eliminate the problem.
Just my two tin pennies worth.
Have a Great Day, Brent