Have to use TableDefs to modify AllowZeroLength. I have code that creates table with CREATE TABLE and adds fields with ALTER TABLE then sets AllowZeroLength and DefaultValue properties. Here is extract.
Code:
'must use DAO to set AllowZeroLength property, I don't allow zero length fields and Access defaults to Yes
Set db = DBEngine.OpenDatabase("folderpath\filename")
'change text field AllowZeroLength default Yes to No
db.TableDefs("tablename").Fields("fieldname").AllowZeroLength = False
'change number field so 0 is not DefaultValue
db.TableDefs("tablename").Fields("fieldname").DefaultValue = ""