No.

What you'll get is tons of people subscribing to your service, you'll be so happy.

Then they start using your service, right up to that cap, and you're still happy.

Then two months later the chargebacks start rolling in and you're dead in the water.

Curing spam on a service like this will take a lot more than just a message cap, you'll have to think this through very carefully because spammers have been at this much longer than you have.

They'll eat you alive if you're not careful.

ok yes - you need excellent debit/credit card fraud prevention/detection. There are ways to mitigate this risk tho I think, at a cost obviously.