Hi Kranthi,
I had the same problem.This is how i resolved the issue.After the combobox code insert below code
comBoboxName.onAfterRendering = function() {
if (sap.m.ComboBox.prototype.onAfterRendering) {
sap.m.ComboBox.prototype.onAfterRendering.apply(this);
}
document.getElementById("<comBoboxId>-inner").disabled=true;
}
give credits if helpful.
JS Bin - Collaborative JavaScript Debugging
Regards