Hide featured image from post preview

Adding this to your style-sheet will solve your problem.

.attachment-post-thumbnail{
display: none;
}

To edit your style-sheet Appearance->Editor->Select Stylesheet from left side.

Thanks but can I just add it anywhere like at the end?

Yea, sure. You can add it anywhere in the style-sheet.

Nothing happened ??
I added below code after the last } bracket
/* Hide featured Image 17-May-2016 Shyam Uthaman */
.attachment-post-thumbnail{
display: none;
}

Try posting this at the top.

/* Hide featured Image 17-May-2016 Shyam Uthaman */
.attachment-post-thumbnail{
display: none !important;
}

Doesn’t work either

I can see its working here. May be you are seeing the cache. Please do check this in a private window.

Ah yes. The cache was the problem. That was silly of me.
Thank you for your time and help.

You are welcome. Please mark this as resolved.

Thank You