How did Amon deflect bending?

Amon was a highly talented, extremely well trained water-bender of considerable skill, so in any particular conflict he could already block or attempt to block any water attack by a bender of less than or equal skill.

More importantly he was a blood-bender, so he could by taking control of another person's body, alter their posture, thus affecting their ability to bend correctly, altering their pose could deflect their bending attacks as well.

Amon bloodbending Mako while already holding Korra.

Mako's firebending is disrupted after being captured by Amon.

In a conflict where he was fighting both Korra and Mako, he was able to hold Korra still, preventing her from using her bending powers. Dodging Mako's fire-bending, Amon then uses his blood-bending to grab Mako, disrupting his fire-bending ability before capturing both of them.

Amon grabs both Korra and Mako with his forbidden bloodbending techniques.

You can see the battle between Amon, Korra and Mako at this video link. Start around 1:15 if you want to just skip to the fight...