Learnings: Identifying and getting rid of unwanted traffic

I've recently spent some time getting rid of unwanted traffic on my forums and thought maybe the learnings might have value for someone else, so I am writing them up. This is not intended as a tutorial or even advice - it is just a couple of finding that you may find useful or not. Also, there are many ways to Rome, depending from your situation, needs and abilities. So take it with a grain of salt.

Important race conditions for my actions: My forum is pretty small (currently ~2.000...