If we assume that the claim is true, they could've just banned him from the Chat as the TOS says.
That should be the case, but Yobit seems to be unfair and inconsiderate if it's only a troll box violation. It is not the first time I read a post about an account that have blocked just because of that
[same unsolved case posted here and several post about blocked accounts here too].
The way I see it is that they used this as a reason just to block his account and take his funds.
Plus, knowing that they have a poor support, users will just be tired of contacting them and wait for a response.
PS. I never had any problem with Yobit, but if I put my feet in the shoes of these people, that's what I have thought