How do I STOP the auto underline of text in a document?

LibreOffice came with pre-set auto underline in my documents. Let’s stop it!

LibreOffice came with pre-set auto underline

That’s an incorrect statement, hence you need to describe what you are doing. Are you typing something like _my text is underlined_ or did you inadvertently click the U symbol in task bar, or did you modify some paragraph style or, or, or …

Ctrl+U also start to underline text (but without changing text color), Maybe you pressed inadvertently Ctrl+Shift+C that start to record Track Changes (and text is shown in a color similar to “Dark Gold 1”).

Wiggly red lines? That’s automatic spell-checking, deactivate Tools → Automatic Spell Checking (Shift+F7).