First, figure out the Dimensionality of the subtotal. Add Dimensionsality() as an expression. The highest level subtotal is 0 and it goes up from there. If you have a few dimensions, your data would probably be Dimensionality() 3 or something. Once you know the dimensionality of the subtotal, then do something like:
If(Dimensionality() > 2, Expression, ' ')
That will evaluate on your data and any subtotals after the first three dimensions (I believe). Modify that number until you get the results you want. I did a space in the false section of the If, because I don't want the null character in my chart, but instead a blank.
