Munge
I am not letting any random person sign up and get in. I manually have to OK each person, but I am not screening who they are, just making sure they are humans. There are sites to run a search for known evil IP addresses that I check too.
You weren't kidding when you said there will be a lot of spammers, for I constantly have to reject them when they try and sign up. There doesn't appear to be any way to trust that only legitimate humans will sign themselves in and then do human stuff.
I am password protecting even the registered users for access to the downloadable files, and they get it only if they ask for the password. I could further password protect my files within those files if it was really juicy stuff.
This is all to keep the damn search bots from posting everything on the web. I have topos I would like to share or that people have requested that I share to a few people and putting up for all of the web would go against their request so this is a good in-between.
The internet is really evil and invasive.