Set the following two properties:
multiselect: true,
multiboxonly: true,
If you want to click the row not to select, check the checkbox to select it, just override the beforeSelectRow method:
example:
beforeSelectRow: function (rowid, e) {
var $myGrid = $(this),
i = $.jgrid.getCellIndex($(e.target).closest('td')[0]),
cm = $myGrid.jqGrid('getGridParam', 'colModel');
return (cm[i].name === 'cb');
}