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.
Sure nearly impossible to do 100% but if you put enough cats on the job you can make life tough for the mice :-)