What is not clear is this, will players from U.S not be accepted at all ? If people use tor or VPN service to play from U.S what about them. I have read all comments above but didn't find satisfactory answer. This is point to be discussed more clearly.
Yes you will be allowed i guess as in this case its the fault of player and he is cheating in a manner not the site.
If the end user in the USA wants to use the site and goes through the means to access it,and they are using tor or a VPN it would not be the responsibility of the site manager to monitor all proxies.
Simply blocking US IP addresses and using a region block is a reasonable standard of care and would be enough in my opinion.