NetCut and other Arp Spoofers.. How to deal with them?

Hello everyone, i have a network setup, which i own, but i have multiple folk in my building using it, and they contribute to the fees. Recently some have been complaining that there's an Arp spoofer being run on the network. The characteristics are suspiciously those of NetCut since it keeps changing the gateway's MAC address to fake ones.

Now i've been thinking, would finding out my gateway's (modem's) mac address and having everyone associate it with the IP statically solve this? (Using Arp -s 192.168.1.1 [[insert mac address]]) ?

Also, how do i pinpoint who's using the program? Naturally everyone denies even hearing of it.

Jam it back in, in the dark.