Thank you for visiting our forums.
To answer part of your question, yes the FBI can lock computer systems, anyone whom distributes a parasite with capabilities can as well, including you and me.
But, the FBI will not lock computer systems alike the FBI virus. Even if the computer was actually involved in illegal activity the FBI does not send warning, nor simply fine anyone over the internet for such crimes discussed on the fabricated screenshot. If disturbing material such as child pornography is actually present on a computer system, the owner may likely be arrested and will have to appear in court, not fined over the internet.
Hope this makes sense and helps!