I have a request from a landlord friend. He has a bthomehub3 and one of the tenants has been illegally downloading using torrents. He suspects somebody and needs to block their access covertly, to see if the downloading stops and so (think it was OFCOM- it's the DIgital economy bill people) will stop sending the letters warning him. He wants a full block on 1 pc which he knows in bthomehub3 but without access control showing the access has been blocked to check for this (hence the need to do it covertly to confirm if this guy may be the offender)
My first suggestion would be to take the letters to the tenant, or to all the tenants frankly, and ask them politely to stop if they are engaged in illegal activity. Also, you probably don't want to respond to the letters as this will only encourage your ISP to seek legal recourse.
To block one user, and I would only do this as a last recourse, because the tenant is likely to confront you about their lack of internet access, before your landlord can fully ascertain that they are the guilty party. First, discover their MAC address either from the router or with a program like zenmap, than block that mac address from the router.
I understand the landlord's plight. It's something anyone offering internet access faces as a potential threat. However, the truth is, there's almost nothing you can do to block a determined user. MAC filtering? They simply change their MAC address in the driver configuration (takes 60 secs). IP blocking? They use a VPN service (but at least then any legal challenges have to start at the VPN provider and be backtracked to your system; not impossible, but at least another obstacle).
So it's virtually impossible to prevent it using a technical solution. The internet was simply not designed to deal w/ these issues. Every known solution is just a hack, a band-aid that always has a workaround. And anyone engaging in illegal activities is highly likely to be more than capable of skirting around your restrictions.
As G0rd0 suggests, it's a social problem more than a technical one. Bad folks are going to do bad stuff. If someone can be specifically identified, then the only thing that will work is securing access w/ a username/password that's unique to each authorized user. Anything that remains open is never going to be safe from these threats.
If the landlord doesn’t want to be heavy handed, then it’s possible he could direct ALL the traffic through a VPN service (assuming he had a router w/ a VPN client), perhaps spreading the cost among the users. Not a perfect solution, but at least a little more protection.