Stealing ICQ UIN'a

I want to tell the Habrasoobshchestvu about my problem and ask for help.

The icq uin was stolen, six digs, which the admin friend I had presented to me many years ago.

The problem is not as banal as it may seem at first glance. After the password was stolen, I could easily change it using primary email and the www.icq.com/password service. However, the attacker immediately "stole" him again.
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As a result, I changed the password a dozen times. My win does not cease to pass from hand to hand for the second day. I always have antivirus enabled, I tried to log in from other clients, scan the entire system, etc.

I noticed that after each theft, a certain email of the form xxxxMOY_UIN@yandex.ru is registered in my profile. Then I decided to check, and what if I request a password change to this address? And, to my regret, it worked out.

Consequently, the attacker was somehow able to link another email to UIN, to which you can request a password change.

Interested in two questions:

How could he do this?
What should I do in this situation? How to return dear to the heart icq uin?