There is no effective way of ever tracking anyone on the Internet, you can also get tools that will hide your IP address, also if your ISP uses NAT, proxy server, etc. you will share your IP address with thousands of other users.
Also the way HTTP works (thats the protocol that web pages are sent by) is a stateless protocol, meaning that once the page is downloaded the connection to the server is dropped, os the server doesn't know if the person has left the site or is simply reading the page.
This means that there is no effective way of tracking users or being 100% sure who a user is.
You have to get around this by relying onthings like cookies and IP addresses to track users, which is not 100% and can simply be got arund by anyone who wishes.
In the forum you can try to stop the same person sighning up more than once by doing things like turning on email activation, this will only allow an email account to be used by one users when signing up, but again this can be got around by any determined user.