What do you mean with 'de-spawning'?
The easiest way is to create the object outside the viewport and move it into it.
For example:
System| Every x seconds -> System | create object Sprite on layer 0 at (-sprite.width-1, y)
This will spawn an object on the left. -Sprite.width-1 pixel outside of the left border.
If your sprite has the width of 32 it will be created at x=-33, y.
Now you have to move it:
System| every tick -> Sprite| Move forward -1 pixel.
If you want to spawn it at the right side:
System| Every x seconds -> System | create object Sprite on layer 0 at (OriginalWindowWidth+Sprite.width+1, y)
System| every tick -> Sprite| Move forward 1 pixel.
OriginalWindowWidth is the Windows width in the project settings.
This will work as long as the screen doesen't scroll, then you have to use viewportleft and right