Try putting this in your ExpandableListAdapter, listView is a reference to the ExpandableListView itself. And lastExpandedGroupPosition is a integer member variable defined inside your ExpandableListAdapter.
@Override
public void onGroupExpanded(int groupPosition){
//collapse the old expanded group, if not the same
//as new group to expand
if(groupPosition != lastExpandedGroupPosition){
listView.collapseGroup(lastExpandedGroupPosition);
}
super.onGroupExpanded(groupPosition);
lastExpandedGroupPosition = groupPosition;
}