Right now things are calm - we have few or no problem people and few infractions.
Often you can just ban a problem, but things can get complex - sometimes a valued member may do something stupid, or maybe younger players sometimes need to be straightened out. Sometimes you want something in-game, with fewer consequences for lighter crimes, especially as the server population grows.
Here's the question: Do we want a prison? A prison is any place that a player can be sent to where they cannot escape. Normally either the region the prison is in disallows teleports or you demote that player to a "convict" access level (with no /home, /tp or /warp access). A prison can range from a simple box (or pit) that you send (or throw) people into. There are plugins that manage imprisonment (letting them out after days, hours, or minutes), though in our case we can just give mods the ability to release people. You also have to deal with death - it doesn't help if you can off yourself and respawn elsewhere.
If we do want a prison, where do we want it? Ideally I'd say put it in Spawn, in a prominent location where people can see and go to it. Do we want to allow anyone to come by and look at the prisoners? Do we want to give prisoners an actual environment to roam around in instead of just a box with bars? If we build one, we can be purely functional about it (just a place to put people out of the way for a bit) or we can go all out (complete with a prison complex and potential routes for escape - making it a PvE of sorts).
It would also make for an interesting group project.