Has this always happened here or is this a fairly recent thing? I can't answer that. But, what i do think could be the reason for these posts, is they are new people with literally nothing to say, so they write some BS that we've all seen dozens of times, even though the answers are most probably available in the next thread.
This has always happened, however it is now exaggerated by the fact that the forum is more active than it ever has been. The increased media coverage of Bitcoin, and cryptocurrencies didn't help us in terms of the signal to noise ratio. Bitcoin hitting a new high also encouraged others to get involved with Bitcoin, and the ICO boom has also contributed to the issues.
I don't know the extent of how many people join here daily and how many are just timewasters etc, but i suspect a fair few will NEVER participate in any part of the forum other than the bounty threads, and even then it's pretty much guaranteed to be very very basic.
What i thought of was an idea where an Introductory section was implemented into the forum, whereby, if someone were to join the forum they would be pointed to this specific section and would be asked what they are joining for, what they hope to achieve, what can they bring to the table etc etc. Basically, a way to see if people are genuine or not and in a way-POLICE (politely) members prior to actually posting elsewhere.
This sounds a little like the newbie jail which was implemented a few years ago. This was abolished because of the exact same reasons why your proposed changes would not work. They fall into the same pitfalls of requiring unnecessary content that we just don't need on the forum. For example, back when the newbie jail was implemented there was a restriction that you had to post x amount of posts in order to get out, and wait a certain amount of time. I think this was a total of 1 post, and 4 hours respectfully.
What this resulted in is newbies opted to post introduction threads which added no value to the forum other than them saying hello and describing a bit about themselves. I've seen dedicated sections to this on other forums, and they just aren't needed unless someone is introducing themselves but, also bringing an opportunity for discussion. If the thread has a point other than introducing themselves then they are normally fine, but this wasn't the case when the newbie jail was around, because they were forced to make one post.
Now what has been proposed is a quiz which would be asked of new registrations at the time of registration, and depending on the answers given there may be certain restrictions placed upon them. For example, questions related to bounties, and trying to find out through a series of questions whether a new user is primarily looking to participate in bounties. Honestly, this will likely be ineffective once the questions/restrictions are public knowledge, and won't prevent the malicious users that register, and only prevent genuine users having the freedom to do what other users can do.
This may seem harsh to some, but i think its a good way of encouraging the right sort of people to be joining up here and sets a precedent that this place isn't a meal ticket, spam fest or a pathway to riches.
Finally, all this does is confine the spam to one section, and encourage that section to be bombarded by useless posts. It effects genuine members from partipating because they have to go through a real shit period to be able to participate in the forum as normal, and the users that we are trying to prevent spamming up the joint will be fine jumping through holes if it means they can earn money.
I encourage anyone that can come up with a solution to voice their opinion. What we do know is something has to be done, and theymos knows this. He's probably brainstorming himself on the best way to go about it, because let's be completely honest there's no easy way to come at this problem without slamming restrictions on everyone which sucks especially when there's genuine users wanting to participate, but they never get through because of the loops they have to jump through.
) neither will the spam caused by bounties stop. They will move towards the services section, pay 1$ in bitcoin and the rest in tokens. Hell they can even pay in cents for all they care. Even if campaigns as a whole(signature and bounties) go down, airdrops would possibly stay(?). And not to forget giveaways and other stuff in games and rounds. 
when i post in this section as don't want to give the wrong impression of me. I just thought i'd share an observation of what i see here on a daily basis as a newer member than a lot of others here.