In the Market you can choose to mark any comment on any app as one of the following:
Marking something as Spam will remove it from the list of comments, but only on the account that marked it as such (that is, if you view the comments from another person's account/phone, the comment is still there). Does marking a comment as Helpful/Unhelpful have any actual effects? If not, has Google ever laid out/announced any plans to use this feature in some way?
I have seen, with some of my apps that I have developed, a comment be there, and then later on, the comment is no longer visible.
And it's not from me marking it as spam, as I have looked on other devices too, just to confirm.
So it would look like that after "x" number of "spam/unhelpful" votes, it gets removed.
I can not tell for sure but I think that voting helpful and unhelpful affects the sort order of the market comments, at least in the web version of the Android market. I see many comments that are older than the most recent listed on first or second position. Maybe because these comments received many helpful votes.
Marking comments as spam will remove them immediately for you and maybe if the comment receives more spam marks it will be removed completely.
This is my assumption. If you have some official links that explain the functionality let us know. :)