Yes, an ISP can restrict your access to the internet. And they do it.
Here are some examples of how they can do it. I added a quick list at the end of what one can do to prevent their ISP to tamper with their connection.
Also, note that your ISP is not the only component that can implement censorship. Google could be forced to delist results (eg: after a DMCA takedown notice), and any website can be asked to remove content - legally or illegaly.
An ISP can simply have a list of domain names or IP addresses that needs to be blocked. Anytime a request comes to their network for one of these IP address and/or domain name, the ISP can simply drop the connection.
An ISP could also make that decision based on the content of the data transmitted if it is not encrypted.
When browsing the internet, you usually rely on domain name, such as security.stackexchange.com. However, while convenient for you, your computer will need to request the IP address associated with the domain names. This service uses the DNS protocol and is usually provided by your ISP.
This allows them to block or redirect access to some websites considered as illegal upon a court decision. Here's an example of a court decision instructing ISP to redirect the traffic to a website to a landing page.
IT IS FURTHER ORDERED that all ISPs [...] and any other ISPs providing services in the United States shall block access to the Website at any domain address known today. [...] The domain addresses and any Newly-Detected Websites shall be channeled in such a way that users will be unable to connect and/or use the Website, and will be diverted by the ISPs’ DNS servers to a landing page operated and controlled by Plaintiffs (the “Landing Page”) which can be reached as follows:
IP Address: 220.127.116.11 (Dedicated)
Source: United King Film Distribution LTD, et al. v. Does 1-10, d/b/a Israeli-tv.com, 21 Civ. 11025 (KPF), Default Judgment And Permanent Injunction Order (Apr. 26, 2022)
BGP stands for Border Gateway Protocol and is used to interconnect different networks. An ISP can then reroute the traffic to another location, either to make a website unavailable, or to redirect the users to another server. (As seen with the DNS example.)
An example is when the Pakistani PCCW company rerouted traffic to Youtube upon a government request. As the link states, it had unforeseen side effects.
What can I do?
As your trafic initially goes through your ISP network, the main mitigations would be to prevent it from identifying your trafic as suspicious and blocking/redirecting it. A non-exhaustive list of countermeasures could be:
- Not rely on your DNS provider (see below) which can easily be changed in most OS settings;
- Use a proxy or VPN to reroute your trafic to a trusted network;
- Use encryption between you and the service requested to avoid any content-based censorship;
- Switch to another ISP that do not block the content you want to access.
I note that you are using Tor, which should mitigate some of these issues. We would need more information to understand what has been tampered with to make better assumptions on what's happening.