Very irritating for you.
If you can see the email address they're sending from (assuming it doesn't change each time) and assuming you're using Apple Mail macOS....
1. Open Apple Mail
2. Copy the email address of the offending email
3. Go to top left corner and click Mail / Settings...
4. Click 'Rules' icon on top row
5. Click 'Add Rule' button on right hand side
6. In 'Description' call it something like 'Send to Bin'
7. Then choose your parameters... perhaps:
If 'any' (select from drop down) of the following conditions are met:
Then from the dropdowns select
'From' 'contains' and then paste the offending email address (or type it in)
8. Under 'Perform following actions' select from the dropdowns
'Move message' to mailbox 'All bins'
9. Click ok.
Now, every time they email it should simply go straight to your Bin.
And the above reads rather more complicated than it will seem on screen and thus more convoluted than I intended!
Hope that helps,
Adam