Users enforce as layer one. Through voting, reporting to mods. You can @ me in any thread in the communities I moderate, or send me a private message, for example.

Mods enforce as layer two, within their communities. Mod tools are not awesome (well, neither were reddits, so...). I regularly scan through the communities I moderate just looking things over. A good way to find problematic things is to sort by "New Comments".

Mods hosting communities on the server are subject to the rules that the admins set and enforce. So layer three -- if the mods are not doing their job, you go to the admins. Admins admonish the mods, or ultimately, remove their power and/or community.