simple solution for a big problem

we all the know the problem about the possibilty of spoofing the " thing " and using it combined by the ip address to get a free service

i noticed that to get that done the spoofer needs to input the combination of the ip address and the " thing " manually

i.e : if i forced the costumer to use the dhcp, this problem might be solved all the way

is there any way to force all the costumers to get their ips automatic and get nothing if anyone tried to input it manual ?

sorry if the problem is already solved but i did search about a fix for it and got nothing

That is not going to work becase attacker fakes IP/MAC combo of a valid user. Some solutions were discussed here:

http://forum.mikrotik.com/t/32-subnet-mask-to-prevent-arp-spoofing/26921/1