Yes.
Usually you would initialize the control, set some attributes, and add it to your window's content view, but without at least a little bit of help such as maybe OS and Xcode versions, language, exactly what you are wanting to do and what you have done so far, etc, you aren't going to get much help.
NSString *comboEntry = [outletComboBoxCell stringValue]; // get comboBox entry
[outletComboBoxCell removeItemWithObjectValue:comboEntry]; // delete comboBox entry
[outletComboBoxCell setStringValue:@""]; // set comboBoxCell to nul
Can you explain why a combo box is the best choice for what you are trying to accomplish? To me, it sounds like a rather awkward tool for the kind of thing you are describing (editing a list).