This slogan, in all seriousness, will be appearing in magazines and on bus stops in Australia. I don't know what 'dickhead' means in Australian, but around here it means someone who isn't very nice.
Needless to say, being a dickhead is neither a crime nor a likely result of smoking pot. Ironically, however, writing the word 'dickhead' on a bus stop generally is a crime if done for any purpose other than this one.
Given what we know about the effectiveness of absurd anti-drug ads, I'd guess this campaign is unlikely to cause a decrease in marijuana use, but it might cause an increase in the use of the word 'dickhead.'
In fact, it already has.