What have you got so far?

In Processing, the power is enough (in general) to repeat simple operations like that on each frame. So you don't undo an operation, you just don't do it in the next frames...

You don't necessarily blend the images on each frame, anyway, you can do the operation in an auxiliary PGraphics, and display the result in draw(). But to remove the blending, just use image() without blend(), or image() of the background instead of image() of the blended result.