I think the argument that Quickseller is on referring too is that if an announcement thread was trashed for this without offering an incentive to post competitors of the ICO/Altcoin could sabotage other projects like this. Of course, if the replies don't get removed then it would be giving them free advertising and be pretty counter productive.  

I think it's just best to report posts that are bumping the thread individually unless, the team is offering an incentive for posting in their threads and then the announcement thread would then be removed which effectively deals with both issues.