You can block access to any website by adding its domain to the PC's "Hosts" file. The Hosts file has no extension but is plain text and can be opened in Notepad. In XP, it is in the path:
and entries are added by entering the Localhost IP address of 127.0.0.1 followed by a space, which is created using the *TAB* key, not the Spacebar, and then the domain to block. For example, you could block access to these forums by entering:
What it actually does is to redirect the URL for pcadvisor.co.uk to your own computer where it doesn't exist, instead of to a DNS server which will find its correct IP address and open the page.
Never having visited a chatroom and not knowing what software is used to do so, I cannot help with that, but if it uses a website URL, it can similarly be blocked in the Hosts file.