I want hide it completely from the main grid too.
See image for more clarification.
Thanks in advance.
The column which you need to hide has the name "subgrid". So to hide it you can use
$("#grid").jqGrid("hideCol", "subgrid");
If you need to implement interface for opening or closing subgrids you can call toggleSubGridRow method directly. For example the following callback will toggle subgrid on double-click on the main row:
ondblClickRow: function (rowid) {
$(this).jqGrid("toggleSubGridRow", rowid);
}
See the corresponding demo here:

UPDATED: If you want to hide additionally the icon from the first column of subgrid you can use the option
subGridOptions: { openicon: "ui-helper-hidden" }
and add additional CSS style
.subgrid-cell .ui-helper-hidden { display: none}
See the updated demo:

UPDATED 2: To hide the column with the icon in the subgrid row you can just use
.subgrid-cell { display: none}
See the next demo:
