I can imagine two possibilities: The ignition switch "ACC" contacts are
stuck on, or some accessory is feeding voltage onto the ACC wire.

Remove fuse #28. If this does NOT turn off the stereo, there is voltage
feeding the ACC wire from somewhere else. Could be an internal short in
the stereo between the power supply wire and the memory keep-alive wire.

Put back in fuse #28 and remove fuse #27. If this DOES turn off the
stereo, there is voltage feeding the ACC wire from somewhere else.

If removing fuse #28 DOES turn off the stereo, or removing fuse #27 dos
NOT turn off the stereo, it could be a bad ignition switch or there
could be a short somewhere in the wiring between the under-dash fuse box
and the ignition switch.

im not sure but u used to have a domestic car that used to do that, it was
the ignition switch. you should try other items that are on when the radio
is on to see if its the switch.

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Call your dealer. . .Maybe your Civic quallifies for the ignition switch
recall?

'Curly'