You can chemically straighten your currently grown hair permanently. Once new hairs grow it will go back to normal.

Hair shape (curly or straight or somewhere in between) is determined by the follicle, which is itself determined by genetics. Nothing to be done about that, I'm afraid! At least not currently; I'm sure one day in the future we will be able to alter our genes on-the-fly, but until 2077, we're stuck with our old hair-straighteners.

Chemotherapy has been known to sometimes change the curliness of a persons hair... when it grows back.

Yes, japanese hair straightening techniques achieve this. I learned a lot about hair straightening from reading Curly Hair, Curly Girl reddits etc. Because they aim to avoid straight hair and instead train locks to maintain curls, they actually talk a lot about what will cause hair to remain straight.

Hair grows from the root. You can get a relaxer that will permanently straighten the hair but the roots will grow in curly. It's pretty hard on your hair though, and generally speaking you still have to blow dry it straight to get it smooth.

(If you are talking about your own hair - learning to take care of it can make curly hair an absolute joy to have, and you can still get it blown straight on occasion. I find that the best approach.)

Back in the day I would comb a perm solution through my hair. I’de continue to comb throughout the specified time and then rinse/condition per the box instructions. I recall being happy with the results. Not sure if they still sell at home perm kits, though.

Your hir growslall the time. In order to permnently straighten your hair, u need any newly grown hair to also be straight . since yor hair type is contolled by DNA, you would need to alter DNA of your hair follicles. Its tecnicaly possilebuft would be extremely expensive

Varnish.