@JigokuShoujo666
He is not logging into your account. He is doing failed logins with an incorrect password.
You can file a ticket with IMVU customer support, and they will ban his IP ... but all he has to do is use a proxy to start doing it again.
There is no real way to stop it. Just wait for him to get tired and find a new target. There is a limit to how many people he can do it to at the same time.