To fix your issue, you will have to get the Dimension control and remove the string you dont want to see.
In my dialog() method I have added this line,
dimension = dialog.addField(typeid(SysDimension)); // Add the SysDimension - Department, Cost Center & Purpose
Override dialogPostRun and get the control and delete the string. In my case, I want to remove Purpose string.
public void dialogPostRun(DialogRunbase dialog)
formRun = dialog.formRun();
formComboBoxControl = formRun.design().controlName(dimension.name());
Hope this helps.