I'm no expert on bus topology, but I'll do my best to answer this:
All frames/packets in the bus network will have a source and destination address. All systems in the bus sniff every frame and ignore the ones that aren't destined towards them. Most of this work can be done by a decent NIC.
Where my system is connected to a bus, I will see:
Message 1: Destination: John, source: Sheila (I'll ignore this)
Message 2: Destination: Sheila, source: John (I'll ignore this)
Message 3: Destination: Me, source: John (I'll read this and respond if necessary)