What is the highest diopter that would disqualify a person from getting LASIK/ PRK?
There really isn't a "hard line" beyond which laser vision correction should not be done -this depends on many factors that can only be evaluated in the course of a thorough ocular examination and biometric testing. This is true with any kind of laser procedure of the cornea: LASIK, PRK, SMILE, etc.
Laser vision correction was originally approved by the American FDA for up to -12 diopters. However, this was in the early days of LASIK and we know much more now than 25+ years ago. In general, the higher the degree of (myopia) nearsightedness, the greater the risks. This is because there is a proportional relationship between how much myopia is being treated and how much thinning the procedure will do to the cornea. Thin = unstable over the long term. I rarely treat over about -7 diopters with any of the laser vision treatments, but that number is not hard and fast.
There are other kinds of treatments for high myopia that may well be better and safer that do not work on the cornea at all.