you have to delete identifier in the parameter with name "newParameter" Directon="In" for the method used for item creation.
for example:
<Method Name="Create">
<Parameters>
<Parameter Name="returnParameter" Direction="Return">
<TypeDescriptor TypeName="AAA.SampleEntity, AAA" Name="returnParameter">
<TypeDescriptors>
<TypeDescriptor Name="idUnitaDiMisura" TypeName="System.Guid" ReadOnly="true" IdentifierName="idUnitaDiMisura"/>
<TypeDescriptor Name="dsUnitaDiMisura" TypeName="System.String" />
<TypeDescriptor Name="dtCreated" TypeName="System.Nullable`1[[System.DateTime, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]" />
<TypeDescriptor Name="CreatedBy" TypeName="System.String" />
</TypeDescriptors>
</TypeDescriptor>
</Parameter>
<Parameter Name="newParameter" Direction="In">
<TypeDescriptor TypeName="AAA.SP_UnitaDiMisura, AAA" Name="newParameter">
<TypeDescriptors>
<TypeDescriptor Name="idUnitaDiMisura" TypeName="System.Guid" CreatorField="true" IdentifierName="idUnitaDiMisura"/>
<TypeDescriptor Name="dsUnitaDiMisura" TypeName="System.String" CreatorField="true" />
</TypeDescriptors>
</TypeDescriptor>
</Parameter>
</Parameters>
<MethodInstances>
<MethodInstance Type="Creator" ReturnParameterName="returnParameter" Name="Create" ReturnTypeDescriptorPath="returnParameter" />
</MethodInstances>
</Method>
in the newParameter I removed dtCreated and CreatedBy