Sorry to hear that, but it's called the iwillbackupmysave branch for a reason, stuff like this unfortunately happens.
Couple things on this, it has been considered but that's work (creating user names, pws, taking applications, etc), and honestly I'm quite lazy. That and who knows if this was something as malicious as cheating, yes when losing game time style work due to someone else its frustrating and the first thought that crosses our minds is probably something along the lines of "hackzorz". But this could have been someones honest mistake, forgetting they had an active mod and logging onto mp is not that unreasonable. Going by the server list it looks like some are modding the map, I can see someone server hopping onto a wrong server and botching something easily. Main reason I'd rather leave it public is new players. When pz went on sale over the weekend the server got completely flooded with connection requests, which yes caused issues with stability but it was still freaking awesome just knowing lots of people wanted onto an open server. I don't want to basically turn the server into a closed community. If this keeps happening I will make the switch over to private, but for now holding off on it.