But I expect the "probation period" proposal also to be effective with respect to spam reduction by multiple accounts. The reasons are:
1) like I already wrote, the "probation period" per account, when spamming becomes dangerous for your profits, is potentially much longer than the period needed to get a single merit. This should cause almost all spammers to be "caught" before they can really profit, and thus the period to profit from a single spam account would be very short. Most spam accounts would be caught before the first payout.
2) spammers that change the account must also re-apply to bounty campaigns. This is probably not easy to be done automatically. If you must re-apply for every 0.0001 BTC (or so) you could eventually get if you're not caught, the incentive to do that isn't very high.
These two effects should drastically reduce the incentives to create multiple accounts. It may be not so effective against "manual spam" than the Jr. Member Merit requirement, but regarding copy-and-paste bots it's maybe even more effective.
@cryptovigi, I forgot to reply you earlier:
"Admin Why did I loose my signature?"
"Somebody reported me of spam which I didn't - please admin check it up!",
"Who reported me? Why? Admin help!",
The rest of the "complaints" you mention (e.g. spammer soliciting data about the person reporting it) is simply illegitimate PM spam prohibited by the forum rules, so admins can ignore it and even ban the offenders if the problem becomes an issue. This should be clarified in the PM about the deletion ("Don't PM the mod or you will be banned"), and there can be an "official thread" for complaints, but without any obligation for admins to take action as the punishment is not harsh at all - you can start a new account or collect 10 merits.
While my calculation based on this olde thread (at that moment there were about 400 merits generated/day by merit sources, so I guess now it should be at least 600, which results into a total of ~1200/day) is even more optimistic than yours, I am a bit pessimistic about the effective 30% sent to Newbies. AFAIK it was near 10-15%. The other problem is that users often "hoard" merits, intentionally or unintentionally, so many Merits simply are "lost".
In the last months, only 100-150 merits per day were effectively distributed (the strange thing being that this is much less than what Merit sources generate according to the thread I linked). One of the new Merit sources I know is limited to 10 new sMerits/month for now, so I guess this should be the normal rate for "beginner Merit sources". That doesn't add much, considering that these days about 40-50 new Merit sources were added.
Anyway, I see this proposal has not been very popular. I won't insist on it. But I'll observe the Merit stats and if I see that Merit distribution isn't improving, I may bring it back on the table some day.