How to get the default value to not show when selecting it from a picklist.

We have areas on our isometric drawings that pull values from an OpenPlant model by usinga picklist through an SQL query. Purchase spec, install spec, series number, area, etc. On most of these queries, there is a default value placed in the schema, such as < Select Purchase Spec >. In the model, under pipeline information, the section for Purchase Spec will say < Select Purchase Spec > when the pipeline manager is opened, and when we create the isometric, that node on the border will be empty. We use the Node2da list to fill in these areas on the drawing or in the bill of materials.

The value on the drawing will be blank until the user selects an item from the list. This works exactly as we want.

The problem is that if the requirement for that purchase spec or other item changes, the only thing a user can select from the list that isn't a number is the < Select Purchase Spec >. The next time the isometric is created however, instead of having a blank spot for Purchase Spec, the drawing actually shows < Select Purchase Spec >, the Default value.

Is there a setting we can add to the schema that will make the default value blank again in the drawing if something has already been selected from the list? I have added a "" to the picklist that will create a blank line in the pulldown section for purchase spec but it wouldn't be obvious to a user to select the blank line to have nothing show on the drawing. We are looking for something that will make the < Select Purchase Spec > show when the pipeline is first generated but won't put that text in the isometric if it's selected later.