Disable British to American spelling Autocorrect feature

Set the text to be in the “English (United Kingdom)” language.  In recent versions of Word, this is accessible by going to the “Review” tab, “Language” pane, “Language” button/menu, “Set Proofing Language…”.

         

You might want to click on “Set As Default”.

The specific example you give - organise vs. organize - is not related to the proofing language. Both are acceptable spellings in British English, and while the -ise version is most common in British English some style guides - most particularly the OED - use the -ize version.

If you want to have your spell checker reject the -ize version you'll need to add it to Word's exclusion list. This is a file, found at:

C:\Users\KennyLJ\AppData\Roaming\Microsoft\UProof\ExcludeDictionaryEN809.lex

You'll have to replace KennyLJ with your actual username. The file contains a list of words that will be marked as wrongly spelled even though they are in the British English dictionary.

You can edit this file with notepad and add organized, organizer etc. to it, one word per line.