not able to delete thumbnail??

Hello,

If you have disabled images in the form, and “Featured Image” also is disabled, then that pretty much stops the plugin from adding or doing anything related to featured images. So if you are still seeing a thumbnail after disabling both of those settings, it must be some other plugin or script that is doing it. I recommend troubleshooting your plugins and theme, here is a complete guide:

https://perishablepress.com/how-to-troubleshoot-wordpress/