Restrict the Shipment Types drop down list on shipment screen set

Dear All,

I am having a requirement to restrict the drop down list with only few values for the filed shipment_type .

When we try and edit the screen set that does not give any option on what type we need to keep and what needs to be restricted .

I just want 2 fields that is REPOSITION , SECONDARY CHARGE in shipment type selection.

How can this be achieved ?