Using the MAC address as a condition for session termination in general
Generally a changed MAC address might be used as a weak condition to terminate sessions, but be aware
- they can be arbitrarily faked for most network cards (so you just need to find one which makes this possible)
- they will also change when a user is on the same device, but switches network interfaces (for example between mobile broadband, WiFi and Ethernet connection).
Servers do not know MAC addresses of clients
Anyway, learning the MAC address is not possible for a server, as the server does not receive the client's MAC address at all and cannot query for it (as long as he's not on the same network segment, which will pretty much never happen in any internet scenario).
MAC addresses are only used for data transmission on OSI layer 2 (Data link), and MAC address information of the previous sender is discarded by each router on the path as he replaces the frame headers for the next hop.
The Address Resolution Protocol ARP will not help at learning the client's MAC address either, as it's limited to the local network segment.