nodes – Why can not an ISP prohibit access to a blockchain network?

I read that blockchain is at the top of the Internet and is decentralized. We still have to access the blockchain network through an ISP. Without the ISP, we can not access the Internet, why can not an ISP completely block access to a blockchain network?

Many have talked about p2p but without ISP, how can we connect to other nodes? Is not it an easy feat for an ISP to block blockchain?