Oh dear, people are trying to be creative and have some fun, beyond just posting a YouTube video without description or discussing random stuff for a page and a half.
We definitely need to put a stop to that.
Seriously, there's nothing wrong with people doing things like these.
It's just a bit of fun and it lets people try and be creative.
It's a community mystery game. Why aren't there more things like that?
I personally want to see more mysterious community games and things. A forum-wide Cluedo game, where 1 member is subtley infected and has to pass it on or something, giving very subtle clues in their speech as they go about the forums.
If it really gets on your nerves, then why don't you simply skip over the topics?
Sure, the titles might not start off with "Playing an ARG in here", but once you've clicked on it once, you're readily capable of pressing backspace and leaving again.
The only time that it's even credible to complain about multiple topics is when those topics are breaking the rules, or cross-posting, or happen to be identical. Just of note, these things aren't identical, even if they're the same sort of thing.
Two fan-threads for a TV show would be having useless identical topics.
For everything else there's always something else on these forums.
If an ARG is unsuccessful (just of note Maxx, try planning it a lot more in advance) then the topic will die. No need to worry about it.
If its successful, then you just ignore that one topic out of the many out there, or you can decide to enter and join in on the fun.