The answer would depend on the context.
If you're building a thing such as a bulletin board, it makes sense to show “Content removed”-type message when a post was removed by the original author, while keeping the remaining part of the discussion untouched—doing otherwise would put the whole discussion at a mercy of the original poster, which is rarely a good thing, given that the discussion could derive a lot from its origins.
If you're building something where a reply to a post doesn't make sense without the original post, then the removal of the whole discussion makes sense. For instance, on Stack Exchange, it would be really weird to keep the answers to a question which was removed. Nevertheless, you should probably design a system where authors could still recover their replies even after the original post was removed. One of the annoying aspects of Stack Exchange is specifically the fact that a few months after you've spent half an hour writing an answer, you see the community (or the OP) remove the question, and it's very difficult to get the answer in order to copy it somewhere else.
Side note: if you're talking about nested comments, you're probably reasoning as a programmer. There is nothing wrong in reasoning as a programmer, unless you do it to design user interfaces. Hierarchical structures are a wrong way to show information in most cases; hierarchical bulletin boards are an excellent example of an unusable interface—unusable because being designed with programmer's logic. Unless the design was created by a professional user experience designer who is absolutely sure that hierarchical approach is the way to go, please, please, avoid this approach. There are already too many low quality user interfaces in this world.