In one of the best studies seen in a while, Nathan Matias shows announcing the community rules increases rule compliance by newcomers by 8%+ and the participation of newcomers by 80%.
“An experiment tested these theories by randomizing announcements of community rules to large-scale online conversations in a science-discussion community with 13 million subscribers. Compared with discussions with no mention of community expectations, displaying the rules increased newcomer rule compliance by >8 percentage points and increased the participation rate of newcomers in discussions by 70% on average. Making community norms visible prevented unruly and harassing conversations by influencing how people behaved within the conversation and also by influencing who chose to join.”
Instead of forcing members to mindlessly tick a code of conduct, email them the clear rules which makes your community unique.