Okay, I've been thinking a bit about the whole barriers solution thing.
I don't think it's possible to have metal barriers (like those they have for the last K) on all the spectator hotspots, pretty sure I've seen them on the last K of a "big climb" as well, but what about the rest of the climb?
My suggestion is a simpler, cheaper sollution for that: stick some poles into the ground and tie some rope between them, creating a barriers (think they used it at the Arenberg during PR). This way people would know that they aren't supposed to go further out than the barriers allow, but it wouldn't make it feel as much as if they were "fenced of" as metal barriers would. I honestly believe that the majority of people running next to the riders doesn't do it out of any sense of malice, but rather from a feeling of:
"OMG! This is so awesome!" (and being drunk).
If a person jumps a barriers (metal or rope) anyway, well... Fine them!