Is there a way to hard factory reset my iphone without passcode, itunes, or computer?

I found this iphone (5s I think) and I was wondering if there's a way to reset it w/o all that junk.

Nope. At the very least you'll need a computer with iTunes to install a fresh operating system over USB. You may still have trouble if the (ahem) rightful owner has locked it down.

It's really easy. Allow the rightful owner to do it by handing the phone in so it can be returned.

"Finders keepers" has no legal validity, but "Theft by finder" is a very well established legal principle. That iPhone is permanently useless to you and as soon as you turn it on it could well be reporting its location and even sending covert photos of you and your surroundings.

In short, it's not YOUR iPhone so you can't do anything with it except hand it in.