What about having her bury the parking spot in snow while he’s gone? For second offense, she could bury the parking spot while his car is in the parking spot.
If she “buries the parking spot in snow while he’s gone”, then that means that she will have to dig out that parking space once again upon her return in order for her to be able to use that space. How would that benefit her?
And, if she buries the other guy’s car, besides shoveling additional snow she is definitely inviting retribution.
I don’t recommend either of your suggestions.
If she were willing to shovel snow back and forth, why wouldn’t she just clear a new space for her car and resove the problem?
Burying someone’s car in snow would escalate the problem, not resolve it.
After living in Buffalo, NY, where parking spaces are available on a first-come-first-served basis, I think this whole idea of blocking off parking spaces is selfish and lazy.
I also live in Boston (JP) and nothing annoys me more than the stupid snow-parking wars. It’s New England. It snows. Get out your shovel and get over it. I would never in a million years think that I owned a parking spot just because I shoveled it, no matter what Mayor Mumbles says. I’ve never saved a spot and I never will.
That said, I would also never risk putting my car in a spot that some jerk has blocked off with trashcans or whatnot because that’s just asking for trouble. He knew what he was getting into. Have at him, Nicole, but only if it’s within the legal 48 hours 
I don’t really think there’s anything TO do.
Since she left two notes, he’d clearly know who it was if anything was done to his car, and (from limited information) he seems like he’d be enough of a jerk to escalate. And there’s also the fact that Nicole, being female, would be susceptible to physical intimidation (granted, she might be stronger than the transgressor here; I’m just playing the percentages).
I also can’t believe that Boston actually went and codified “reserving” a parking place! This is a time-honored tradition where I’m from, but it’s merely a mutually-agreed-upon concept: it doesn’t carry the force of law.