Not just ISPs, it can be blocked at the enterprise level in a few clicks.
I was temping at a place during the pandemic when my hospitality based IT job shuttered. With their set up, I could just block a country in a couple clicks.
I didn’t do the clicking, but we were getting hit with a DDoS from a nation we had no business in, and it was just blocked in a matter of minutes once the meetings and BS were attended to. Those took hours over days.