Removing specific methods from child class which are inherited from parent class

The code is as below, just the basic structure:

class FooType(type):
    def __new__( cls, name, bases, classdict ):
        instance = type.__new__( cls, name, bases, classdict )
        # What can I do here?
        return instance

class FooBase( object, metaclass=FooType ):
    def __init__( self ):
        pass

class Foo( FooBase ):
    def __init__( self, name ):
        self.name = name

    def method1( self ):
        pass

    def method2( self ):
        pass

    def specialmethod( self ):
        pass

class A( Foo ):
    pass

class B( Foo ):
    pass

class C( Foo ):
    _disallowed_methods = ['specialmethod']

What I want to do is that instances of class C should not have specialmethod, but that method should be available to instances A and B.

I can override this method in class C and raise an error, but I would prefer not to do this.

I realize I can add in code to check for _disallowed_methods in the FooType and on the basis of that check if the instance has any of them in the output of dir(instance). But I cannot seem to remove the method from __dict__ of C using any methods I have tried so far. The methods I tried are delattr(instance, 'specialmethod'), and del instance.__dict__['specialmethod'].

The delattr method results in "AttributeError: specialmethod", and the del method results in "TypeError: 'dict_proxy' object does not support item deletion"

Basically many different classes will inherit from Foo, but some of them should not have specific methods available to them like C which should not have specialmethod available to it.

What am I doing wrong? Or how else can I accomplish this?