Undo/Cancel in individual vote
Head over to relative path
/admin/structure/rate, and use the edit link for the rate widget for which you want to be able to Undo/Cancel a vote.
id corresponding to the widget you selected), scroll down to the section titled "Interaction". There you'll be able to change the value for "Should a second click on the same button delete the vote?" from "No" to "Yes". Save your changes, and repeat for each of the rating widgets for which you want to enable this facility.
Set or delete votes for selected roles
There doesn't seem to be a Drupal interface for this. However, you may want to use the Services Voting API, which provide Services endpoints (actions) for Voting API module. From its project page:
You are allowed to get votes as flexible as Voting API allows it.
You are allowed to set votes as flexible as Voting API allows it.
You are allowed to delete votes.
From its README.txt:
After enabling the module as usual...
- Go to admin/people/permissions
- Enable the following permissions for your desired user role(s):
- Retrieve votes
- Set votes
- Delete votes
- Go to admin/structure/services
- Click 'edit' next to your endpoint
- Enable the following resources under the 'votingapi' service:
And the issue about How to use this services resources? contains an interesting (JSON) samples about how to actually set votes*.