I've had more success with Apple Mail on my iMac. By interrogating the headers (View/Message/All Headers), and taking the domain number that follows the final 'Received', I have set up rules that work (Mail/Preferences/Rules/Add Rule). First I needed to add 'Received' to the list of message headers that appears in the drop-down menu within the Rule dialogue. To do this open the drop-down (defaults to Any recipient) and choose 'Edit header list..'. In the Message Headers box that appears click the + symbol and type 'Received', then OK.
Now using the Rule dialogue add any domain number that follows the final 'Received' in the Headers and choose a mailbox to which to divert the messages. I created a new mailbox for this purpose. Be warned, some non spam messages my be diverted, but my experience is that these will be few, the majority being spam. This is fine as far as it goes, but of course it only diverts the spam, and doesn't allow you to send an undeliverable, for example, so it does nothing to dissuade the spammers.
In my case I'm lucky that the address being spammed is one I had been using only on my iPhone and iPad, and then only for new mail. I have been able, therefore, to disable it on these devices and use an alternative.
Back at base I can use the iMac to check the diverted spam and delete it en masse. Not a perfect solution but at least it's under some sort of control.