Initialize combo

I can o populate a combo with certain values based on value selected in previous combo.


However i found the values (in Combo Box) keeps on getting added ..here I want to initialised the combo prior to populating the same .

How can I do it..
Please help

I am using fgollowing code
which just adds value in combo box everytime I call it...

Help me to initialise prior to caling this loop/routine

for (var i = 0; i < jarr_subid.length; i++)
if (mid == jarr_mainid[i])
{
selObj.options[selObj.length] = Option(jarr_subname[i],jarr_subid[i]);
}