You can only delete risks if your workshop has not data. Even if you have not voted on the specific risk that you wish to delete, the software will not allow any deletions.
To deselect the item, so that the software does not open votes for the risk, on the list view, click on the cell where the item letter is shown. This will change it from green (selected) to white and Matrix will skip it from the voting process and all graphs.
You may wish to double click the risk description on the list to open the item editor add a note of why the item has been skipped.