User blog comment:B214/Volume 21 info & spoilers/@comment-83.7.233.159-20160420144354/@comment-1359322-20160425093336

"……Maybe I can’t reveal all of them. I’m thinking it will be great if I can introduce them on another project that is based on the same World. [Dragon-Eater] Samael is the ultimate anti-dragon. Ophis and the Two-Heavenly Dragons were crushed by its curse. But that is just the problem of affinity and it doesn’t necessary mean that he is invincible. Except Samael is a forbidden existence that can make Dragons extinct. Even Georg who is a Longinus possessor and who is a splendid magician can only control him for a short time even though Samael has many restraints on him. Naturally Samael is something that is sealed so this is the end of his appearance."

- Volume 11 afterword

Also in that volume itself, Ishibumi wrote this:

"“The one who turned into a snake and made Adam and Eve eat the fruit of knowledge is that. That action brought the fury of “God from the Bible”. So God started to hate snakes and dragons to the extreme. That’s the reason why dragons are depicted as evil in many of the texts from the Church. He is a being that was made of the evil intentions of God, because of God’s hate, a being who received many kinds of poisons and curses on his body. Naturally it is impossible for God, who is sacred, to have evil intentions. That’s why it has intense poison. Not only can it extinguish dragons but it can affect other things besides dragons, so it was sealed in the depths of Cocytus."

So no God can't control Samael. Samael is a product that he produce out of rage hence is uncontrollable.