De-activate an iphone 4 after restoring it?

I bought an iphone 4 online very cheap, when i got it it had a passcode, so i restored it to factory settings but now it is asking me to activate it by linking it to the apple ID, but i bought it online and the previous owner is not answering my emails. How can de activate it and activate it with my apple id without using his.

You don't. Until you get a hold of the owner, you own a brick that looks like an iPhone.
He will need to delete his Apple ID from the device on his iCloud account, that is the ONLY way this will be happening. And something that he should have and you ask to have done before money traded hands.

It would be a good idea for you to have the phone's ESN or IMEI checked, to see if the phone was reported lost, stolen or is still being financed or has been defaulted on for payment. All of which means you got scammed and you will never hear from this seller again.