You don't have to worry about getting rid of ss's, they're only temporary in the dwg. When you close the dwg, the ss collection is cleared. All you have to worry about is when creating a new one, you don't try to use a name that already exists or you'll get an error. The usual method of avoiding runtime errors is to error trap. Try to create the set with the name you want. If it errors, reuse the name.
Code:
Public Function AddSelectionSet1(SetName As String) As AcadSelectionSet
' This routine does the error trapping neccessary for creating selection sets.
On Error Resume Next
Set AddSelectionSet = ThisDrawing.SelectionSets.Add(SetName)
If Err.Number <> 0 Then
Set AddSelectionSet = ThisDrawing.SelectionSets.Item(SetName)
End If
End Function
Once you have the ss, use ss.Clear to start fresh.