long story short, i have two good ideas.

Firstly, let's make the app, and only in this app you can send the solutions in the ongoing contest, but on web version you cannot send. And you cannot login to the app while contest is going, so if he registered from two accounts, he cannot log in to the second while contest is going(i think app is not necessary if it's possible to make it even with web version).

Secondly, let's ban not account but whole device, like in some games. And like in Faceit we can create accounts only with ID/Passport. If Hes cheated his rating is decreased and for first time he's banned from participating in contests for 3days, after it for week and etc.

It's very insecure method, but i think the amount of cheaters will be decreased by 90%.