How to "clear" the downvalues of a subscripted variable?

Question

I have a function that sets certain subscripts of the symbol aa as in the following example:

listA = { Subscript[aa,2], Subscript[aa,1,4,7], Subscript[aa,2,4,5,8] };
listB = {1,2,3};
Evaluate[listA]=listB

This causes Subscript[aa,2] = 1, etc. Later I wish to reuse the function, sometimes with some of the same downvalues of aa. But, first I must unset those downvalues.

I know I can clear ALL subscripted variables using Clear [ "Subscript" ]. But I need to clear only the subscripted values of the symbol aa while leaving other subscripted symbols alone. Is there a way to do something like the following?

Clear [ "Subscript[ aa ]" ] or Subscript[aa,u__]=. or
clear downvalues of aa

I suppose I could consider learning the Notation package, but all my functions involving subscripts are now working. I am just having this problem of re-using them during a single session.

Answer 1

This will remove all DownValues of Subscript that contain the symbol aa:

DownValues[Subscript] = Cases[DownValues[Subscript], dv_ /; FreeQ[dv, aa]]

It won't remove SubValues, but those can be removed in a similar way.

You can avoid the trouble with DownValues by using UpValues, as shown in Clear complains that a subscripted variable is not a symbol or a string?, or in How do I clear all variables with subscripts?