Freaky guest keeps coming back!

For a very long time now, I have had alot of guests come onto my site, for a while it didn't bother me, then I started checking the IPs because I was getting freaked out. And I found someone come on twice in a row in pratically the middle of nowhere. So, I was extremely freaked out, I banned their IP, then they came back! Yes with a different IP but it was ALMOST the same as the one I banned! With like 1 number change! I'm started to get scared about this guest. Is there anything I can do?

I guess you could always make it so that only members can view or post. But if they are just viewing, whats the harm?

I am currently working on making the forum unviewable to guests. But I've never had the same guest come back that many times =\ That's what's making me scared because I keep thinking they might be a hacker or something. =\