Are you sure that you have the correct permissions to edit the list (those can be found under the groups and permissions, select the group to edit and then select the permissions tab and the corresponding tab for the module to be changed and then select the options as desired)?

Is this a customized workflow, as the default list for the Defect status is the Bug_Status list which does not contain the ''N/A'' status as an available option by default?

You could as an option hide that field and not use it and create a User_Defined defect field with the available options you would like displayed based upon a custom list that you design.

See if this helps. Here is a link to an Orasi KB article that demonstrates how to omit a status code from the list.

[ How to omit Defect status from drop down list (CLICK HERE TO DOWNLOAD)][1]

[1]: http://www.ftporasi.com/EyeOnTesting/QC/Workflow/Workflow%20Example%20-%20How%20to%20Hide%20AndOr%20omit%20a%20XXXt%20Statuses%20in%20ALM%20v11/Workflow%20Example%20-%20How%20to%20Hide%20AndOr%20Remove%20a%20Defect%20Status%20in%20ALM%20v11.pdf