The only thing that works against spam is to destroy its economic utility. Perhaps, instead of a free-net, what's needed is a for-a-nominal-charge-net? Sounds

The only thing that works against spam is to destroy its economic utility. Perhaps, instead of a free-net, what's needed is a for-a-nominal-charge-net? Sounds like you want hashcash (http://hashcash.org http://hashcash.org).