Can't help but picture a friend trying to explain the cheesecloth or flypaper on his doormat. 
I used motion detector sprinklers for one cat problem. My yard was the local litter-box, and I was tired of arriving home to wonderful new smells. I used the same sensors as in "motion lights" to sense movement, then output 110vAC from that to a 24vDC transformer, and from that to a 24V sprinkler valve which supplied two nozzles (two directions). Worked great for my flower-bed & garden. Not sure you'd be happy with it spraying your front door. Perhaps you could cut off his avenue of approach? There are simple versions that run off batteries and a garden hose.
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There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195
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