in places like the US most airfields are owned by the municipality? They are public places, like a bus station.
Yes, many airfields in the US and Canada are municipally owned, but they can still be closed.
Any competent authority can close the municipal airport (or the bus station). This would include the operator, the police, the fire department, and Transport Canada.
For example, near me, CYOO, Oshawa, Ontario.
From the Canadian Flight Supplement, in part, (my bold)
OSHAWA
ON
CYOO
REF
N43 55 22 W78 53 42 Adj N 11°W
UTC-5(4) Elev 459 ́ VTA A5000
F-21 LO6 T2 HI5 CAP
OPR
Muni 905-576-8146 Cert
1130-0330Z‡ O/T PPR
The airport is public, and open from 1130 to 0330 UTC. The competent authority (the Airport Manager - Stephen ( I know him well))
can close the airport, and does every night.
So when it is not open, it is closed. But, with prior permission you could land there. Only an air ambulance, or police aircraft will easily get that permission. That permitted aircraft would have permission to land, but the airport will still be closed to everyone else.
Meigs field, where I used to fly as a destination from time to time, was also closed.
Meigs Field - Wikipedia, the free encyclopedia
To be sure there was no misunderstanding, they dug up the runway one night, and the remaining aircraft had to be trucked out. No one liked it, and the authority who closed it was not regarded as competent, but it was closed.
"Closed" is an absolute term, not subject to misunderstanding. Operation only by exemption, or opening.