Is there a way to put a command in my LyX document preamble to inhibit numeration of theorems, propositions, and definitions in the whole document?
If you already have a numbered Theorems module loaded, just
Definition* from the drop-down menu at the top left.
I have the Italian version:
If you don't have a numbered Theorems module loaded, go to
Document -> Settings -> Modules, choose
Then, from the drop-down menu at the top left, you can choose