Lemmy.ml, the most popular instance fo Lemmy out there, is not accessible in my country unless I use a VPN. But, I can subscribe to any community that exists on lemmy.ml from here and even post on those communities from this account.
I love this aspect of Lemmy.
This will be nearly impossible. Popular instances can be blocked but someone can simply create a new small one each time. It is eternal cat and mouse. Censorship by the country where it is hosted however will be effective for that instance but no other.
Sure nearly impossible to do 100% but if you put enough cats on the job you can make life tough for the mice :-)