Copy and paste your text to a new document without the original formatting. This really sucks and can be time consuming to reformat but it is what I do as a last resort. Save it with a different name. You could do a google search asking this question and maybe get some bettet advice.
Do you need to do the check as you go, or can you not do it all at the end as you revise and edit?
Personally I hate having it on all the time as I grew up in Canada and the US so often mix words ending in re/er while I type. I also find the constant reminder that I have typed soemthing wrong distracting if I am trying to get a document done in a certain time frame.