How does one denote that a cell value is not available / doesn't make sense in a table?
If it is self-evident (remember to look at your table from the perspective of a reader) I would just leave the cell empty. Less clutter is better. When not, I would tend towards --. More importantly I would add a footnote to the table explaining what -- means. I think the footnote is in that case more important than the difference between -, --, and ---.