[alphamerik]

Have you considered charging for write access? Allow the site to be publicly readable but charge people a yearly fee to submit stories and posts. They would be much less likely to act abusive if they are paying and their access could be revoked. Several membership levels and tiers of forums could be provided, including private and invite only.

[blasdel]

No need to make it that complicated — just require a one-time $5 fee to create an account.

Metafilter has done this very successfully for the last 6 years.