Generally, once a JPG has been opened in ACR and has an embedded adjustment
recipe it will continue to be opened in ACR even though you have the
preference set to not prefer ACR for JPGs.
Your verbiage is different than what I see for Camera RAW Preferences in ACR
5.4 on Windows, so maybe you need to update to the latest ACR or maybe Mac
and PC versions of ACR are different.
In my CS4 Extended (11.0.1) / Edit / Preferences / Camera RAW preferences
for ACR 5.4, down at the bottom, you can set JPG and TIFF Handling - JPEG to
Disable JPEG Support instead of Automatically open JPEGs with Settings to
make it ignore the settings in JPEG and/or TIFF files.