Currently, it is not possible to disallow pasting images into the Rich Text Control specifically. You may wish to submit a feature request for this on our Nintex Product Feedback site and make sure to upvote it:  https://nintex.uservoice.com/forums/932677-k2-five

Hey @SumitSu,

Even though you cannot block pasting images, you should be able to implement validation on your Next/Save/Submit button to check if an image was pasted in the RTE by checking if the control contains "<img". And then you can throw an error saying you are not allowed to have an image in your text.

You can use this same logic for any other unwanted rich text element.

HTH,

Ernie