How to remove every insect from romaine lettuce?
Meet the common aphid. These little insects suck sap from your salad and are totally harmless, should the occasional hitchhiker slip your attention.
To remove them, there are a few home remedies:

Soak the leaves in salted water (a tablespoon for a large bowl should do).
Or use a few dashes of vinegar instead.

Both soaking methods can be improved by gently agitating the leaves every now and then and if you use a bowl and let it overflow instead of dumping the lettuce in your kitchen sink, the aphids that float on the surface get carried away instead of reattaching themselves when you lift out the leaves.
Except for really heavy infestations (can happen to the most diligent the organic gardeners), I often simply rinse the little critters off the individual leaves under running water.  Think of your faucet as a “mini pressure washer”: Don’t hold the leaves right under the faucet, but lower and let gravity help you. Then even a little water is surprisingly effective. In my experience lukewarm water works better than cold water, but I can only speculate why. Maybe because cold water makes the aphids “stiff” and they can’t let go? The most stubborn ones come off with a gentle nudge with your finger, fingernail or (if you are especially squeamish) a knife tip.