Following JavaScript function will show you how to sets a cell from the selected row not to allow editing.

function SetNoEdit()
{
    var grid = ISGetObject("WebGrid1");

    if (grid.GetSelectedObject() == null)
    {
        alert("Please select a row.");
        return;
    }

    var row = grid.GetSelectedObject().GetRowObject();
    var cell = row.GetCell("ProductName");

    if (row.Type == "Record")
        cell.SetForceNoEdit(true);
    else
        alert("Please select a Record Row");
}

The SetForceNoEdit(boolean) forces this cell not to allow editing. If parameter is null or not specified then the default is true. If this is set to false, the cell would be editable.

Check out this WebGrid live sample which demonstrates WebGrid's client-side API. Click the "Set City as not editable for new row only" button and see how it works.

Hope this helps.