There is a multiple login mod that is floating around here or on threadsdev that we wrote that uses cookies to identify multiple usernames from the same computer. I believe people have kept modifying the code so it works with the current version of threads. I'd install it and use that to collect evidence.
We have, unfortunately, been through this cycle far more times that I'd even care to admit in the last 10 years. The range of personalities who do this stuff range from people with a chip on their shoulder to severe mental health issues.
You can call the police, but there is little they can do unless you have a very solid case with serious credible threats they can build off of. Nowadays I think they take this more seriously, but about 5 years ago they definitely did not have the capability.
Really the best way of handling this is putting your pride aside and diffusing the situation as quickly as possible. Get the wrong person and this will go on for YEARS. Ask them very directly, in a non-inflammatory way what their issue is, and then ask them to either respect the rules of the forum or go away. They likely will not. Like Sirdude said, ban them and move on.
