they have two neutrals in 12 months . . . . can't see that that would do too much damge to their business.

I would respond to their last email with:

"I appreciate your distress at receiving a neutral feedback rating from me.  I thought long and hard about giving you a negative, but decided on a neutral instead.

I do not appreciate your recent emails requesting I revise my feedback . . . . in fact, I want them to stop.  If you persist with sending me emails I will add a followup feedback to my original feedback stating that 'seller is bombarding me with unwarranted emails, not appreciated', and as you have already left folllowup you will not be able to respond to that.

I am sure that would do more damage to your business so please stop emailling me, and I will leave it as it is".