I believe that as a regular user (not an admin account) who has been flagged in SFS, posting and sending message is blocked. So even if a spammer gets in through an error page, they are blocked from doing a bit of interacting.

Current developer of UBB.threads PHP Forum Software
Current Release: UBBT 7.7.5 // Preview: UBBT 8.0.0
isaac @ id242.com // my forum @ CelicaHobby.com