MMNavController.h

@interface MMNavController : UINavigationController

@property (nonatomic,readonly) BOOL canPushOrPop;

@property (nonatomic,readonly) id navLock;

@end

MMNavController.m

@implementation MMNavController

#pragma mark UINavigationController Overrides

-(void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated {
	if ( self.canPushOrPop ) {
		[super pushViewController:viewController animated:animated];
	}
}

-(NSArray*)popToRootViewControllerAnimated:(BOOL)animated {
	if ( self.canPushOrPop ) {
		return [super popToRootViewControllerAnimated:animated];
	}
	else {
		return @[];
	}
}

-(NSArray*)popToViewController:(UIViewController *)viewController animated:(BOOL)animated {
	if ( self.canPushOrPop ) {
		return [super popToViewController:viewController animated:animated];
	}
	else {
		return @[];
	}
}

#pragma mark PUBLIC PROPERTIES

-(BOOL)canPushOrPop {
	id navLock = self.navLock;
	id topVC = self.topViewController;
	
	return ( (! navLock) || (navLock == topVC) );
}

-(id)navLock {
	return self.topViewController;
}

@end