Any use of fecal matter towards another person is not only a Health, Life, Safety issue. it's also a Hazmat concern. You should contact the property manager, HOA and local LE.
BUT...................... Do each contact by Registered mail. This way there's none of this, well we may have received something, but our records don't show any contact . WE also had an issue with our computer and lost all correspondence
You need to get through to any entity you contact the potential for serious health issues from said fecal matter is very real.
Next time there's some form of "fecal matter" on your steps, deck, anywhere other than the lawn. Call the FD. Explain there's a questionable substance and you have concerns it might be of a hazardous nature.
Threatening you with any form of bowl matter IS a form of Bio-terrorism. See how well the HOA and others react when you inform them.
REMEMBER YOU ARE THE VICTIM.