That's not too big of an issue. I have a script for compiling the report results which I can easily send to yahoo so that he can see the amount of spam that each individual member generates.
By 10? I doubt it. You'll most likely have those bland two/three-liners that are vague and padded enough to just barely count as substantial.
I'm pretty sure it would help him manage this.
If the threat of losing payment is not enough they could also add some other "incentives" like getting banned from future campaigns managed by yahoo, red trust... At some point they'll see that it's better to earn $10 a day safely than risk losing it all + your account to get a few dollars more.
That said, 20 posts a day is a crazy high number that's like made to cause spam and chaos on the forum. Most people barely do 20 per week.