If you change your signature, it's still some advertisements. That is to say, the number was stolen, maybe the landlord's computer was infected with a virus, or he was hit by a Trojan horse when surfing the Internet.

It is suggested that the landlord modify the password to improve the security level of QQ, for example, set login permissions for your own QQ, so that you can log in to your QQ in some areas but not in other places. My QQ is like this, and I have never logged in from a different place.