Until one knows the reason someone may have parked that way, one should not take action. This is what's wrong with every strand of vigilantism. Sure, it could be simple inconsideration, but that usually isn't the case; some other believable causes for the behavior could be simple ignorance (which calls for education, not vandalism), or emergency circumstance (which makes stickering the only antisocial behavior in this case), just for examples. There is no good reason to forgo the benefit of the doubt. On an emotional level, OP isn't acting in a "mature" way, or being brave, or a force for good, or anything of the like. This behavior is cowardly, passive-aggressive, and slightly criminal, and sooner or later will likely end in what I can only see as a much-deserved beat-down.