Actually, in my Osclass 3.7.4 with 1 category price disabled when logged in and publishing a new ad in this cat the price field is *not* displayed to me. After posting the new ad and going back in edit mode for me the price field is still not being displayed which means it is working as intended as far as price field goes.
What is not being hidden is the price currency selector and the price label, this should probably be fixed in JS (easy fix) since it makes sense not to display anything related to the price when it is disabled. IF you have disabled price for items in admin (not category) the function discussed here will hide both price label field and currency selector.
If your price field is displayed when editing an item in a category with price disabled there might be somethingelse going on related to your code/theme. The JS used to hide the price field is the same for publishing and editing.
Regards,
Eric