Why such users cannot be banished without the right to restore accounts. There are rules of the forum-it's like a Constitution. Who its violates the ban. Social networks don't stand on ceremony - broke rules - have a rest. Here it is time to enter such system long ago-users arrive every day to garbage becomes more and more.
They are banned when spotted. The trouble is they have hundreds if not thousands of accounts stockpiled and ready to use, and as Loyce said bellow you can't really win a war against automation. The biggest weapon we have against them is a simple captcha and that can obviously be bypassed very easily. Without putting in more hurdles to jump through you can't really do anything about it, but that just annoys or penalises everybody else in the process (and bots don't get annoyed).