If you want to clear memory used by this sprite:
Sprite destroy
Wait 0
System unload unused images from memory
If you need to keep the sprite, you can load 1x1 px image into it instead of destroying:
Sprite load image from URL: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII="
You need to do this for every frame, and then unload imaged.