how to block huge traffic

i recently faced a problem and it still exit … i receive a huge traffic from my LAN and it access the WAN it’s very huge that do not anything else run i monitor it in torch .. i can find out the source of this traffic to block it but all i can do is to block the destination port , in this case the WAN not congested but LAN is .. .. but the problem is that each time the dest change at the first 16628 tcp, now 0 tcp but i notice that it’s unicast desinated to random address allways ended by 0
x.y.z.0/32 how can i block x.y.z.0/32 no matter what is x.y.z is ??
Prob.JPG

Find you local LAN IP and disable the gateway, or if you use manageable switch… turn off the port

Well this is not possible as i dont use manageable switch in addition i can’t locate the source of this traffic and my LAN is huge