When you enter text, LibreOffice automatically recognises a word that may be a URL and replaces the word with a hyperlink. LibreOffice formats the hyperlink with direct font attributes (colour and underline) the properties of which are obtained from certain Character Styles.
If you do not want LibreOffice to automatically recognise URLs as you are typing, there are several ways of turning off this feature.
Undo URL Recognition
When you are typing and notice that a text has just been automatically converted into a hyperlink, press CommandCtrl+Z to undo this formatting.
If you do not notice this conversion until later, select the hyperlink, open the context menu and choose Remove Hyperlink.
Turn off URL Recognition
Load a document of the type for which you want to modify the URL recognition.
If you want to modify the URL recognition for text documents, open a text document.
Choose Tools - AutoCorrect Options.
In the AutoCorrect dialogue, select the Options tab.
If you unmark URL Recognition, words will no longer be automatically replaced with hyperlinks.
In LibreOffice Writer there are two check boxes in front of URL Recognition. The box in the first column is for later post-editing and the box in the second column is for AutoCorrect as you type.