If the feds can misuse this clause "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes" to ban marijuana and other drugs (even those that never leave an individual state), they can surely misuse the second amendment "regulated" to ban guns.
Which is what Giuliani was saying; that you can't ban guns, you can only regulate them, and any such regulation must be consistent with the Second Amendment which allows people to keep and bear arms.