WordPress automatically discovers enclosures you reference in a post. Use this plugin to stop WordPress from doing that.

Adds your post thumbnails to your feed as enclosures.

Add image media content and itunes:image tags to RSS

The RSS Image Feed adds the first image of a post to your feeds, even in firefox and even if you only display the excerpt.

Adds the post’s featured image to the default WordPress RSS feed. Optional Media RSS/enclosure, fallbacks, and emoji cleanup.