After taking a look at the services offered on several internet websites, I do agree with the merit requirement for juniors will be a great step ahead against the spamming problem.
Take a look at this one, for example:


This guy is offering "bumping" services on your ann thread. In fact, it seems to be profitable for him, for he has managed to get some clients. If you pay attention to the description, the guy claims to have more than 200 JR Member accounts.
Now, if all those juniors accounts had needed 5 merits (or at least one) to become junior, then he will manage 200 Newbies accounts and be able to do the same.
So, maybe implementing a merit to become Newbie and 5 Merits to become Jr is the solution.
In addition, if newbies and Juniors CAN'T bump a thread, this guy "enterprise" will be pointless.
I don't think that guys like this one are rare. In fact, one of the most troubling information is how he manages to have 200 accounts
The link to his "services" (he has many different offers):
- ICO marketing for $100: https://www.seoclerk.com/content-writing/602427/Cryptocurrency-ICO-marketing-on-Bitcointalk-and-other-crypto-forums
- Bump your thread for $100: https://www.seoclerk.com/Traffic/611685/Bump-Up-Your-Bitcointalk-Thread

So, do you think that by asking for merits to this kind of accounts will be a good solution?
To me, it is. It will be difficult for this guys to sell the services, I hope.