Some comments:

A friend told me that she once got her mother to park in a visitor spot opposite a tradie’s garage which he used as a workshop and for storage.  When they returned, they found the must luridly obscene and threatening note on Mum’s windscreen, the gist of it being to never dare park on “his” parking spot again.

Does their scheme have the standard by-law against offensive language against individuals on the common property?

What constitutes a visitor?  Is it someone who drops in for an hour for a cup of tea?

Definitely yes

Is it the romantic partner of a resident who arrives on a Friday night, parks up, and leaves on a Monday morning?

On the margin of acceptability

Are the family or friends who bring four cars to an airbnb holiday apartment that has only two attached car spots “visitors”?

Unacceptable

How about a resident who parks for 15 minutes while they unload their shopping?

Acceptable as long as they don’t linger.

Or half a dozen tradies working on a renovation who are there all day and every day until it’s finished?  They’re not residents, so what are they?

Acceptable but unfortunate.  There are strategies for filling the spaces after they leave mid-afternoon if you are expecting visitors next day!

Enterprising tenant renting beepers to other buildings’ workers.

Words fail me!