Kick out everyone who earned less than 15 Merit in the last 120 days. Check the merited posts, to make sure they're worth the Merit. That should largely reduce the amount of spam.
Then, increase pay rates for the good posters, keep a public list of participants, and check their posts before paying. This is a lot of work, ask for a raise if needed.
Seriously
You should know that merit system does not work as it should be. Most of the merit revolves around the
Gang members .
I have seen plenty of Jr Members, Members doing superb posts and getting no merits. However, Gang members do give 1 Merit to Newbies to become a Jr. Member and 10 merits to a Jr Member to become a member and consider they have done the job.

Don't give me 10 example where a person move from Jr Member to Hero......and ignore the 1000000 cases where he can't move up

You people think everyone else is mad and only you are wise.
Check the merited posts, to make sure they're worth the Merit.
First of all merit is difficult to get, even if a person get 15 merits in 120, you still want him to be checked if the posts were worth merit.
The merit which you have received till now are all worth it ? I would suggest you make a job more easier for Steve is to give the list of people which should be included in the campaign and also the high rates, so you can post few posts a week and enjoy the living..

I agree and have pointed out the same: a lot of the merit simply goes to those who have already accumulated merit - it doesn't necessarily flow to quality posts. I have been a professional writer and use proper grammar and spelling, and make sure my posts have substance, and I rarely get merit. Right now I am interacting with the thread and am not just posting to post.
A requirement of 15 earned merits would exclude me, and it's not clear it would favor those with more quality posts. The merit just does not flow around. People can argue differently, but so be it.