I can think of two potential meanings for this phrase. The most common, and the first one most people would probably think of, is "[some action] is just not an acceptable thing to do" used to explain why someone should not be doing something. It often has to do with some sort of cultural taboo. For example, a child might ask his or her mother why he or she can't blow his or her nose at the table. The mother could reply with "It's just not done," and the child would take this to mean this is just something that isn't acceptable to do, although there is no particular reason other than custom. Most people don't use this expression very often in everyday life - it's a little old-fashioned and sometimes can make you seem snobbish, as it is associated with the upper class. Note that you can't take out "just," or the meaning changes to "not finished." However, you can change things around to say "It just isn't done," "Those kinds of things just aren't done," etc. Another possible meaning of "It's just not done" would be "it's not finished, but I don't have a good reason why (or I don't want to explain it to you); it just isn't." For example, if a child might ask his or her father why dinner isn't ready yet. If the father doesn't want to explain all the reasons why he hasn't had time to finising preparing dinner yet, he can say, "It's just not done."