I'm not using DHCP; IP addresses are assigned manually, but I'm involved in server leasing, and the management of the leased servers belongs to the customers. Each server is allocated one IP address, so I'm running only one VLAN. How can I solve this using a different method?
1. are you in some kind of rack rental environment? datacenter?
2. if yes, then it (customer gateway spoofing) hardly/has little effect to draw traffic or getting more bandwidth because the amount of interface bandwidth available is obvious.
3. you can try to use separate vlan for the customers, so they don't talk to each other.
4. you can try to implement switch port mac security. limit by 1 macaddr per port.
5. or you can try to use separate pppoe profile and separate pppoe server for each customer.
hope this helps