Perhaps this is the most unusual question I've encountered in my time as an expert on the second installment of the Heroes series.
Do you want to disable autosave to prevent it from overwriting previous autosaves? In that case, I recommend simply renaming the previous autosaves. You can do this either in the Heroes menu (load the desired autosave and rename it) or by simply changing the file name.
Or is there another goal?

Орден Феникса – харизматический :) лидер