sencha hide rowexpander on some rows

One thing is to remove the +/- icon, which can be done via grid viewConfig:

getRowClass: function (record, rowIndex, rowParams, store) {
     var yourFieldofChoice = record.get('yourFieldofChoice');
     if (yourFieldofChoice == null) {
         return 'hide-row-expander';
     }
 },

Define css for hide-row-expander:

.hide-row-expander .x-grid-row-expander {
visibility: hidden;
}