Are both sentences OK?
Someone did something bad and I punished him.

If I say;
Did you think I would let you get away with that?
Did you think I was going to let you get away with that?

Both are fine. I can't see any difference in meaning and you could use either.

I would say number 2 sounds more natural because it is in past tense. I would say either works, but number 2 is my preference. 

Sorry for any inaccurate information, hopefully I helped!