Questions based on FTL travel or infinite forces can't sensibly be given answers that match the requirements for a hard science answer. Instead they've got to be answered with refutations of the central point, and possibly a soft science alternative or a limit treatment of the question.
Some questions however can start with an impossible premise (or at least a premise so unlikely as to be impossible) and have precise physical consequences that are [Hard-Science] through-and-through.
Is it OK if a question is tagged with [Hard-Science] if the question includes a starting premise that is not hard, but which requires a hard look at the consequences of that event?