Hello,

As you are the owner of the website, you should be able to get the IP address of that person who is attacking other members of your website…

With the IP address just go to ARIN or RIPE and search its IP address there, you’ll see which ISP is using that person and also an abuse e-mail address so you can contact with his ISP this way if you want.

Anyway, don’t think that the other ISP will be able to make anything to this person, the ISP can as much tell the customer, and if the attack that he is doing make troubles on the ISP network, then block this customer but for a single website and not doing illegal stuff (physhing, porn, …) the ISP has nothing to do so it’s better that you block this user IP range which will make impossible for this user to login or rejoin your website using this ISP but no other user from the ISP will be able to join it or visit it, also, the user will be able to login using another ISP or from a cyber-cafe.

Hope that helps.

Best regards,
Carles