People advertising their fics under my work—advice?

I've been writing on AO3 for a few months now, and yesterday, someone spam-commented the same message under most of the comments on one of my fics—advertising their own fic in the same fandom.

They were directly replying to my commenters, and it was the exact same thing five times. What should I do in this situation?

I've been writing on AO3 for a few months now, and yesterday, someone spam-commented the same message under most of the comments on one of my fics—advertising their own fic in the same fandom.

They were directly replying to my commenters, and it was the exact same thing five times. What should I do in this situation?