1) ... 'Format cells...', ... 'Text'. ...
2) Alternatively, type ' (an apostrophe) before you start typing...
Sorry, are you thinking of Calc? Both of these methods have issues in Writer, which I expect we're talking about here in the Writer forum
To turn off the auto formatting, right-click in the table > Number Formatting: OFF (unchecked)
Changing the number format to text may appear to work, but in some cases at least, if you later enter a number that Writer recognizes, it will toss out the text format and go back to auto-formatting. Turning off number recognition prevents any auto-changes for all tables in the document.