I have a puzzle site and its an awful way of cheating. Its okay if only partially, but can it be done?
Something I had in mind was replacing the letters with images, but
I don't think there's a way to disable the feature altogether and there should not be a way to disable it.
However you can ensure that some text will not be found by Ctrl+F by writing it in away that the browser doesn't consider continous text.
Using images is one approach that's relatively simply.
Alternatively you can randomize the letters and re-arrange them with some CSS magic (my CSS-fu is too weak to give an example, unfortunately). For example if you want to "hide" the word "hello", then write out "lehol" with each letter in a separate Note that this (and probably all other working solutions as well) will also break copy-and-paste of the text.