import {toggleClass} from '@grapecity/wijmo';

(...)

  initGrid(grid: FlexGrid) {
        this.setGroups(true);
        this.selector = new Selector(grid, {
            itemChecked: () => {
                this.selectedItems = grid.rows.filter(r => r.isSelected);
            }
        });

 grid.formatItem.addHandler((s: any, e: wjcGrid.FormatItemEventArgs) => {
        if (e.panel == s.topLeftCells) {
            toggleClass(e.cell,"wj-state-disabled")
            }
        });
    }