I’m not 100% sure I understand the problem that you are describing. If you could include screen shots or logs that would probably help.
Also, there seems to be some confusion about ASCII and HEX. All data is bits and the bits can be grouped into bytes. ASCII is just a way to encode the data to represent text. There is no ASCII mode vs HEX mode, the data is always just bits which you can interpret as either HEX or ASCII.
If you want to clear the registers of the previous data, make sure to set “Fill unused bytes” to “Enabled”:
You already have the “Offline Option” set to clear the bytes, but those will only be cleared if the device goes offline. If you want to clear registers in your PLC, you need to do that in programming on the PLC.
If I’m not understanding the problem, please share some screen shots or logs to illustrate exactly what you are trying to explain.