Thanks all for your comments.
To heavy1metal, I thought it would be possible to force via Group Policy something that gives LAN connections priority over WiFi connections when a cable is connected, however I do not see where this is, can you point me in the right direction please? I have 2008 R2 DC's.
By the way, no users are admins, so they cannot change settings to their NIC's without the admin login, so setting up a bridge etc is very unlikely.
Related to this, let's say, your office is in range of a completely separate, unsecured WiFi network, let's say a cafe or a shop. Your users connect their laptops to this. They also connect the the LAN by cable. Surely this presents a problem then? There must be a way around this scenario.