You are correct that Objective-C doesn't allow for truly private methods by it's very nature, due to its dynamic dispatch system. However, assuming your question is not about true security, rather simply making it difficult to use the framework in an incorrect way, you have a few options.
A simple, common solution would be to put the declarations for methods you don't want to expose publicly in a category in a separate header file. You can still put these methods' implementations in the main implementation file for the class. So, a header with something like this:
// MyClass+Private.h
@interface MyClass (Private)
- (void)aPrivateMethod;
@end
Then, in your own source files where you need to access those private methods, you simply import MyClass+Private.h.
For a framework, you can set each header file to be Public or Private. Private headers will not be copied into the framework bundle, and therefore won't be visible to users of the Framework. You do this by opening the Utilities pane in Xcode (the right-side slide out pane), selecting the header in question, then choosing Private in the second column of the relevant row under "Target Membership".