jQuery Grid Plugin - jqGrid

I have searched and tried mutiple approaches to what I thought was a simple  issue.  I have a grid that has only one editable column and that is a checkbox.  There are a few records that I don't want a user to be able to change so I used the following code:

afterInsertRow: function(rid,rdata,relem){        

 if(rdata.position=='this' || rdata.position=='that')

$('#'+rid+'_myCellName').attr('disabled',true);

},

No joy.  The checkbox remains enabled and can be checked on and off.

I thought maybe this would work in the colModel:

{ name: 'myCellName',  index: 'myCellName', editable:false, edittype:'checkbox', editoptions: { value:'true:false'}, formatter: 'checkbox', formatoptions: {disabled : evalCheckBoxStatus()},width:60},

but I can't figure out how to get the rowID passed to the evalCheckBoxStatus() function.

I was not sure that I was getting a valid cell reference using the $('#'+rid+'_myCellName').attr('disabled',true); selectior syntax in the afterInsertRow function call but if I do this:

var cellRef = $('#'+rid+'_myCellName');

I DO get a valid object reference (but I am not sure if it is valid for what I want to use it for).  

I have also tried a selector like this:  var cbcell1 = $("input[id='#"+rid+"_myCellName'][type='checkbox']");

but I still can't set the disabled attribute.

So, what am I missing?  This seems like it should be easy but apparently I am missing something essential…..I am using jQuery 1.9.1 and jqGrid 4.5.2.