iCloud Blocks my Erase All Content & Settings command

Trying to erase all content from an old iPhone 4s (iOS 8.3/Verizon) that's been taken offline (and replaced with an iPhone 6), but my efforts are repeatedly blocked by "unable to connect to iCloud" prompts. The old offline phone will not let me turn off Find my iPhone either, or even disconnect from the cloud, because it rejects my correct password entries. Why do I need iCloud's permission anyway to deep-six the content on my old phone? And if the phone's offline and no longer able to use either cellular or wifi networks, how am I ever going to erase its contents?