It's like Rock, Scissors, Paper..... Spray
paint is usually stronger than decals. Short of it being very cheesy, or very fresh, spray paint, other than by very careful buffing, any chemcals that will remove oil based paint will also remove decals. Now, if it happens to be a laxtex based paint, ammonia in soap and water may remove it.
BTW: The storage facility doesn't have vandalism insurance for it's customers?
Good Luck