Yes I can do that but the problem is I couldn't find how to enter such addresses manually into my block list until just now when I just discovered how to do it via 'settings'/'options'.
I was confused because when this happened in the past, I found that although my email address would appear as the sender, the real sender's ID would always be displayed if I hovered the cursor over the message entry and it is this address which would then be entered into the block list. I guess this must have been a different technique from what happened recently then.
Thanks everyone.