Resetting Rows/Cols to zero resets all values. If you want to manually do that then you'd have to loop:
Code:
    With MSFlexGrid1
        .Clear
        For i = 0 To .Rows - 1
            .RowData(i) = 0
        Next i
        For i = 0 To .Cols - 1
            .ColData(i) = 0
        Next i
    End With