LibreOffice collects words that you frequently use in the current session. When you later type the first three letters of a collected word, LibreOffice automatically completes the word.
If there is more than one word in the AutoCorrect memory that matches the three letters that you type, press CommandCtrl+Tab to cycle through the available words. To cycle in the opposite direction, press CommandCtrl+Shift+Tab.
By default, you accept the word completion by pressing the Enter key.
To reject the word completion, continue typing with any other key.
Uncheck Enable word completion.