For an individual item select the 'Does not expire' button. You will not be prompted to enter an expiration date for this item again, Prep & Pantry will remember that this item does not expire. You can always change this setting later by simply setting an expiration date.
If you do not wish to ever be prompted for an expiration date, for any item, this can be set through Settings and Options. Go to the Settings tab> Settings and Options> Require Expiration Date> Off. (It is On by default). Again, if you change you mind on an individual item, just set an expiration date for that item.