Can my phone act as WiFi Access Point while Disabling Celluar Radio?
FWIW I discussed a few ways to disable just the mobile connection here for a different reason but with the same result... depending on your Android version (you didn't mention which you were using), it's as easy as selecting the mobile icon in the notification settings to switch it off, or just selecting an invalid APN under Wifi->Mobile networks->Access Point Names or Network Operators.  But the absolutely easiest and most certain way as I suggested is to pull the SIM card while you're not using it.  As long as your device uses a SIM card for mobile (GSM networks do, Verizon may not), you should be good to go-- though I believe 911 calls will still work.  If you want to get really down-and-dirty, you can also root the phone, rename the binary blobs that work with your cell service, reboot, then do the reverse when you're done.  But I think the SIMcard trick should do it :)