Disabling specific elements from a PickList SelectItem

Hi I have a SelectItem with a MultipleAppearance set to PICKLIST and I am populating it with a String array.
This String contains a list of elements of diffrent categories (these elements are seperated by a “category element”) therefore values in this array look like this:

–Fabric–
cotton
linnen
silk
–Metal–
iron
zink
copper

Now once I set this array to the SelectItem I want to disable the “category elements”, in otherwords I want to disable all the elements which
start with “–” and end with “–”

Below is my code so far but I have no idea how I am going to do the disabling process

SelectItem siAttributes = new SelectItem("Attributes_Select_Item", "Tick the elements you want");
siAttributes.setWrapTitle(true);
siAttributes.setWidth("100%");
siAttributes.setMultiple(true);
siAttributes.setMultipleAppearance(MultipleAppearance.PICKLIST);
String[] titles = rs.getViews_Attributes();//returns a populated String array
siAttributes.setValueMap(titles);

P.S. I am using SmartGWT 2.4