Banning IP addresses will not work for most users because of dynamic IP. For example, when using mobile hotspot I can turn on and off airplane mode and my IP will be changed. Additionally, more than one users can have the same IP address if they are part of the same local network, think of college campus networks.

There is no clean solution to this problem. The only one I can think of is to scrutinize new users, abnormal rating changes, unexpected ranking, etc. In addition to that, delayed banning can hurt their morale. If the cheaters are banned immediately, they can just create a new account. But, if you let them put in months worth of effort in an account that gets banned eventually then that is bound to hurt. See https://mirror.codeforces.com/blog/entry/146474?#comment-1309862

This won't work , If two people use the same network, and one is cheating while the other is genuine, banning the IP would unfairly affect both users (both have same IP ). If two user are using same network they both will not be able to login , suppose if students use college network because of one person all have to suffer .

Guess how much costs one phone number, 20 cents per number from countries like Indonesia or Colombia. This won't stop anything;