Formatting the data does not change it, merely how you see it displayed.
If you really do want to delete the time information rather than simply querying it in the right fashion to get the results you want than as
@jdraw suggested you will have to run a one-time update query.
Update YourTable SET MyDatefield = DateValue(MyDatefield)
Once done this can't be reversed. So make sure you really don't think you will ever need the time element.