You can never truely disable color cartridges on a color printer. Each time you print black a tiny portion of the other cartridges will be consumed in the process to help amplify the black that you end up seeing. There is also a small amount of loss due to the constant opening and closing of the toner cartridges that adds up over time.
Your best bet is to continue to print in greyscale and Black and White depending on what kind of an output you are looking for. Perhaps if you can give us an example of how much color is getting used relative to the number of pages so that we can make additional suggestions.