Where can I report an incorrect “exit only” sign on 101 South exit 498B

Caltrans (California Department of Transportation) has a web form for this type of stuff:

http://www.dot.ca.gov/hq/maint/msrsubmit/

If you don't see a good category for it in the dropdown, you can choose "Other" or use another form they provide for things that don't fit the dropdown categories, but even that form seems to have some constraints on its intended use:

http://www.dot.ca.gov/pioform.html

I would just report it as "Other" on the first form, since it doesn't have a category for incorrect signage.