If she's been repeatedly asked and refused to be reasonable, I think the only way to get it into her head without resorting to wholly unnecessary criminal damage is for two of you to park at either end of her car - get somebody to guide you very slowly so that you're both an inch from her front and rear bumpers.
She'll have no option but to come and knock (possibly at several doors, if she doesn't know whose cars they are) and ask you to move.
When she does this, just refuse and tell her that the cars are parked in a way that suits you and you don't care if it blocks anybody else. I'd over-egg it by saying "I know there are ample marked spaces, as well as the four just around the corner, which would make it sooooo much easier for everybody, but you other people don't park considerately, so why should I bother?" You could even answer the door holding a big glass of wine, which 'sadly' means that you can't move the car until the morning.
If the residents of the house she's visiting have told her about it too, it might be worth speaking to them beforehand and getting them onside.
I imagine she'll splutter and panic at first, but then the penny will drop. If not, you might have to repeat a couple of times. Once she realises and apologises, I'd happily move it and say "Look, no hard feelings, but now you understand how extremely annoying it is when people park selfishly, I'm sure you won't be doing it again, will you?"
I don't think she's a terrible person - just young and/or unthinking, and possibly not a very confident parker - but she'll have to learn sooner or later.