On a lot of other forums when you ignore someone, the system basically does one of two things:
1. Makes both users invisible to each other, like neither knows the other exists anymore
2. There is no "you have ignored this user" spam in a thread
I have one particular poster recently who was made aware they were being ignored and now they have made it a point to basically reply to every single last thing I post just about.
When I ignore someone I don't want them to exist period, not see "this user has posted something but you have them ignored" after every single post I make.