I think steemit should be very very careful about not taking minnows for granted. A LOT of people that I use to engage with here on steemit no longer post here and just left without a word. I don't know if they quit steemit but their account has been dormant for a while.

This solution seems rather draconian and I'm not even sure if it would stop spam since commenting would still be free unlike the memo message function which has a minimum 0.001 SBD fee.