The Sony PRS-T1 has a downloadable service manual and careful reading may well show how to disable or remove the Wi-Fi antenna which should totally disable the wi-fi function. No guarantee but looks like a possibility to me. A similar manual is available for the PRS-950.
The simple solution to me is to use a Sony PRS-350 or PRS-650. Both never had any communications capability independent of the USB port to a computer. Both available at reasonable prices and Calibre was originally designed for the Sony readers to replace the Sony support software.
Another possibility is a ebay Kindle refurbisher who sells on ebay, "tccs". He replaced the broken screen on a Kindle DX for me. I am willing to bet that he could disable communications on a Wi-Fi or 3G Kindle easily. Worth contacting him to ask anyway. Here is a link to his ebay site.
https://www.ebay.com/usr/tccs?_trksi...19.m1438.l2754