If you want to remove an offering completely from position statements, in the offering settings, you can turn “Show Position” to off for all the offerings you don't want to calculate position for.
Select the Issuers Module from the left side of your screen and scroll down to the Offerings window. Select the offering you wish to remove from the statement.
Select the Offering Details tab at the top of your screen and click Edit Offering. From here you can toggle "Show Position Calculation" to "No." This will remove the offering from the statement.