How to disable "Access can be package-private" message in IntelliJ?

To disable on a case by case basis, use

@SuppressWarnings("WeakerAccess")

Go to Settings → Editor → Inspections then Java → Declaration redundancy → Declaration access can be weaker rule.
Here you can either disable it at all or select the suggestion options on the right pane:

The inspection rule is "Declaration access can be weaker" and there are two options which can be disabled "Suggest package-private visibility level..."