Have you got an email address for the housing association. Rather than ring them every time, email them and send a dated photo of said car blocking the space. Anyone flaunting HA rules like this is building up trouble for themselves. It's not her space, it's not your problem that parking is so limited in the. Have you got access to someone with a car who could park it there instead, and pretend that they've moved in upstairs?
You could always repaint the sign - a can of white paint doesnt cost much. It's private property, she's trespassing.
Don't fret about upsetting her - you were here first, you are in the right.