they are bringing it on themselves. why should the op have to spend good money because of laziness.
we had a family that used to park in front of our garage, turned out their hand brake did not work very well, to cut a long story short after pushing their car out of the way a hand full of times they got the picture.
some times you have to take matters in your own hands.
maybe im just showing that ive lived on a council estate for all my life
Petty things like this can easily escalate out of control, the fact is people parking opposite are doing nothing illegal as has been stated by the councils responce, it might be inconsiderate, but thats a fact of life however much it might be wished otherwise.
The sensible option is to move your wall to give yourself enough space to not rely on the sense of others
The alternative is to keep leaving notes, getting hacked off when it happens, and learning to live with the hassel of it, those are your choices
If you dont like the latter, choose the former