The forms –ise and –ize are, in many cases, straightforward spelling variants. However, the –ise spelling is obligatory in certain cases: first, where it forms part of a larger word element, such as –mise in compromise; and second, in verbs corresponding to nouns with an -s- in the stem, such as advertise and televise.

My recommendation is to stick to the –ise form and adjust your spelling dictionary to reflect this. You can easily add words with –ise endings that are flagged by Microsoft Word, but then the spelling checker will accept both forms. To remove the –ize forms, these must be added to an ‘exclude dictionary’. This is a dictionary with the same name as your main spelling dictionary, but with an .exc suffix. See the help files in Microsoft Word for details. You can also use the exclude dictionary to specify spelling preferences where more than one form is acceptable, such as ‘focussed’ and focused’ or ‘inquire’ and ‘enquire’ and their derivatives. Microsoft Word is often installed with the default dictionary set to English (US). This should be changed to English (South Africa) or English (UK).