How to unflash Iphone from sprint back to verizon?

I have an iphone 4 that was flashed to Sprint that i want to put back on Verizon. HOW DO I DO THIS? I've tried factory resetting and erasing all settings and data.

You need to find a Verizon dealer that is willing to flash it… But generally they refuse

if you are not under contract, you can get a new iPhone 4 for dirt cheap directly from Verizon.

You can't, you can reset it all you wish but resetting a phone will NOT unflash it.