Paywalls can have the opposite of the effect you want. Implemented incautiously, they can fail to disincentivize parties who can make profit in excess of the cost, and it can succeed at disincentivizing genuine, non-profit-motivated interaction.

Imagine how much less you would use text messages if they still had a per-message cost.

I would reply to your comment, but my 2GB data allocation for my cell phone is already spent this month.