I will never understand why people believe they own the street in front of their homes. Also, you clearly know who is writing the notes to you, so it would probably be more productive to put this in their mailbox. Or obtain information from the City of Portland that makes it clear that they don't own the space and they need to get over themselves.

Homeoccupier needs to put in a Parking Meter
and/or Restricted Parking signs up --
Assholes Parking Only, eg.

and then perhaps toll HIS street
or, even better
put up a
Gate.

Welcome to Portland and all it's passive aggressiveness! I had the same thing happen to me where I live on a cul-de-sac where all the fuckwads park not along the curb, but perpendicular to the curb. My mom had been watching my toddler 2 days/week and parked in front of the house with some double-digit IQ (being gracious here) redneck with Montana plates and truck nuts. He left a note saying to never park in front of his again otherwise he would forcefully tow my mother's car with said micro penis lifted truck. One visit from the sheriff's office fixed that right up and Mr. Montana had to actually register his vehicle which had out-of-state plates for over a year.