Prevent I have not found a way.
But killing is possible

The shopkeeper maintains his HP across each encounter - so you can wear him down over multiple encounters.

He has the same attack pattern each time - Steal, Panic for 30 block, attack 5 times (with strength increasing by 1 each encounter), Steal, Flee.

Thorns is very useful against him for doing lots of damage quickly.

I can usually do about 50 damage the first fight, and kill him somewhere in act 2 or 3 depending on how great my deck is.

When defeated, he drops 300 souls plus whatever he's stolen (I believe, not sure on that).

Once he is defeated, the rest of the merchant slots will just be the heart merchant, you won't have to fight him again.

Thanks for the help!

There are methods to inflict stun that will enable you can extra turn, but stunned enemies resume their pattern from where they left off.