I did a couple of searches on this topic and didn't seem to find anything so here it goes.

I am running several ccr routers over several sites (WISP) that hand out DHCP at each site. Most clients don't go outside the norm of plugging our equipment into their routers and only pulling 1 ip address from our DHCP server. However, there are a few customers who decide to plug their equipment into a switch and thus they pull multiple IP addresses from us.

How can I restrict them to just 1 IP address per customer?

Not sure of how you distinguish between your customers and how you manage them, but what you are talking about seems to me like a MAC binding use case. Maybe this will help

http://wiki.mikrotik.com/wiki/User_Manager/MAC_binding

Or, a semi-automated option, where you will need to collect the MAC somehow beforehand

http://forum.mikrotik.com/viewtopic.php?t=88359

Last but not the least, this article has something that you may find useful - the "Add ARP for Leases" feature, where you would be able to probably collect or restrict the list later

https://rbgeek.wordpress.com/2013/01/02 ... nd-method/

Please post your findings here