We have spam filters for the community forums, dunno if they extend to game feedback as well. Either way, they have to be updated all the time, and spammers still find their way in. Worse, they tend to latch onto a single target like that and pound it.

Until a better answer comes in, maybe you could disable comments on that game for a few days, then turn them back on?

We do have a spam detector on all comments, but it's possible it hasn't been trained on those types of messages yet so it's not detecting them. 

I've changed the way reporting posts from game pages works, they now get sent to our admin queue, instead of only being reported to the page owner. We'll be going through those and cleaning them up as we see them (going through the backlog now so it might take a bit before I get to your page)