What are synonyms of "I won't spare you"? Is there any phrase less harsh?
Thank you.

Question

Answer

"Spare" means save, to not hurt.

Maybe
1)  "I won't go easy on you" -  I won't treat you in a gentle way, I won't be lenient with you.
2) "I won't let you off" - let someone off = give no punishment

Answer

I agree with Rena. It needs context. Kevin was correct in one sense, but spare can also be 'not kill'. There is a very big difference in meaning!

Answer

You should provide a context for when you read/saw/heard this.  It will be easier to provide a synonym.