Assuming you already found the Tools -> Autocorrect options (which is exactly where you'd expect it to be), that still leaves the (very annoying) autocomplete function enabled. To turn it off in calc, you need to disable:
Tools -> Cell Contents -> AutoInput
This will turn it off until you turn it back on. I found it thanks to
this weblog (update: link is dead).
Unfortunately, when editing a table in Writer, some auto-correct options still remain: numbers are automatically "corrected", +1 and 1.0 for example change into "1". This can be changed by
setting the "Number format" (in right-click menu) of all cells to "Text".
You can probably change the default table style to fix this forever, but I have not tried yet.
(Why do the bloody programmers always think they know better what I want to do than me? If I type two capital letters that is because I want them. If it was a mistake I can correct it myself. Stop forcing this unwanted "automatic" guessing:
90% 99% of the time the guess is wrong!)