Is this ICO open to US investors, given it's a "utility" token rather than a "security"?
Probably not, since basically every ICO out there prohibits US investors from participating, utility or security ones.
but how to close? I don't understand - block ip?
Well, in this case in theory - everyone can use VPN server
You can't close it. If people from the US really want to invest, they can and will invest, you can see this at every ico. But that does not mean that companies shouldn't do their part in preventing it.