"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

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!

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