How to delete comments in Google Spreadsheet?
Added comments can be removed from the cell using the following steps.
Step 1: Mouseover the cell which contains the comments. Click ":" option near "Resolve".
Step 2: Select "Delete" option.
Step 3: Confirmation dialog box opens up to get user choice. If you click "Yes", then the comment will be deleted. Else cancel the current operation.
Step 4: Comment will be removed from the cell and the mark will also be removed.