Put a cheap router between them and have two subnets (such as 198.168.0.* and 198.168.1.*), or if your gear is capable, change the routing in it to implement this. The ARPs will not find their way off the arduino's subnet to the other one where your phones and tablets are connected.
I'll hedge a bit and say I'm pretty sure that will solve the problem, but not 100%. ARP is OSI L1 and therefore does not pass through the router.