Restricting certain public APIs/classes to users in java

I have a application where users can write their customizations using the exposed public APIs. There are few public classes and API that are public but we don't want them to be accessible in the customization code for the users. Is there any way in java to do this without changing the access modifiers of the remaining APIs. I am exploring java security manager, don't know if it would work, any help would be appreciated.