I don't think you need the harpoon. Hippies are usually malnourished vegetarians so you'd probably just miss them.
You might try covering a vw micro bus in fur and drive that to the meeting, bonus points if it's seal fur. Also, to really throw a scare into them, tell them that their patchouli oil is made from north slope petrolium oil and due to peak oil they are going to quit making it! A petition to outlaw dreadlocks due to health codes would be great but would probably get you killed.