i have seen /22 subnets without this behavior something has to be wrong
The problem could be that the provider offers proxy-arp on their router, and the customers do not bother to set
the default gateway in their router but rely on that proxy-arp. In that case, they will probably run in performance
or memory problems, but maybe they do not notice. You are the victim because you see all that arp traffic too.
However, even without proxy arp this is not a good setup. When some other user is not online and gets a
serious amount of incoming traffic, there will be constant arping for his address (especially with a bad router
that does not have some form of arp rate limiting).
It may be that someone is a victim of a DDOS attack and has decided to shut down his router to sit it out,
and then you are confronted with the arp traffic that results from it.
Again, when an ISP makes deployments like this there is little you can do.
(other than pointing them to the weaknesses of their "simple and elegant" solution to metro networking)
The first thing to do is make a trace using the packet sniffer and find the details of the arp traffic. Who is
(claiming to) sending it, what address are they arping for? Is that inside or outside the subnet?
This info will bring you towards the cause of the problem and maybe a solution.