Is this possible

[Kewlz]

If a user enters ICQ number during registration, he'll be directly moved to banned usergroup instead of registered usergroup.

Tl;dr?

If icq value is true
then user's group = group id

[dragonexpert]

You'll need a plugin for this. I believe the hook you will need is datahandler_user_insert.

[Kewlz]

Well just found users can't inenter ICQ number on registrations after that they'll have to enter .
So is it possible if user enters number he'll be banned.