But the impact is already visible on our sales.
Correlation does not imply causation.
Most likely just a coincidence, don't worry about it.
If he spams your threads, make them self moderated and delete any spam.
IIRC, you can't make threads self-moderated after creating them. There is an option when you create it only.
Trust ratings aren't moderated so you can't ask mods/admins to delete them.
Yup, you can't make a post self-moderated after it has been created.
Though, his existing post has mostly spam from this person, and making a new self-moderated post wouldn't be too hard and wouldn't cost him anything.
On trust ratings, you're almost right. The only case where they are moderated is when they are spammed with some other purpose. I think they are moderated when there is advertising in the trust ratings.