Imagine you got a network to which other users can connect with their PC's, you have set up some rules, for example there is a DHCP server that gives addresses 192.168.2.100 - .254 and first 100 addresses are reserved for people who want static IPv4 address.

How is it technically possible to prevent someone from taking some other IPv4 address? How could you restrict someone to use only a specific IP?

How do for example ISP enforce people to use only that public IP they got assigned? For example I have a server in server room and I was granted N IPv4 addresses, I received a list of all IPv4 addresses that I am allowed to use (they are all public IPv4 that can be accessed over internet). I didn't try to take any other IP than I am allowed because that would likely violate some rule and they might shut the server off so I am not even going to try that, but just out of curiosity, is this even technically possible to restrict somehow? How?