Is it possible for an ISP to block access to Tor network or block access to websites accessed through Tor network/client?
The Tor project provides a list of entry nodes, so they can block those by IP address. But then there are bridges that aren't in the list that can act as an entry node. But if the censor is using DPI (Deep Packet Inspection) to examine packets and block those that look as if they are Tor, then a user can try Pluggable Transports aka Meek that obfuscate the packets.
You might like to watch this video: https://www.youtube.com/watch?v=GwMr8Xl7JMQ
How governments have tried to block Tor [28C3]
Iran blocked Tor handshakes using Deep Packet Inspection (DPI) in January 2011 and September 2011. Bluecoat tested out a Tor handshake filter in Syria in June 2011. China has been harvesting and blocking IP addresses for both public Tor relays and private Tor bridges for years.
Roger Dingledine and Jacob Appelbaum will talk about how exactly these governments are doing the blocking, both in terms of what signatures they filter in Tor (and how we've gotten around the blocking in each case), and what technologies they use to deploy the filters -- including the use of Western technology to operate the surveillance and censorship infrastructure in Tunisia (Smartfilter), Syria (Bluecoat), and other countries. We'll cover what we've learned about the mindset of the censor operators (who in many cases don't want to block Tor because they use it!), and how we can measure and track the wide-scale censorship in these countries. Last, we'll explain Tor's development plans to get ahead of the address harvesting and handshake DPI arms races.