Prevent customers to add themselves as Approver

Situation:

1. The reporter opens an issue with approvals;
2. The reporter select himself as an approver;
3. The reporter approver his own issue.

Expected:

1. The reporter opens an issue with approvals;
2. The reporter select himself as an approver, but he can't see himself in the list since his name is already in the "reporter field";
   OR
3. The reporter select himself as an approver;
4. The reporter try to approve himself but receives and error saying that he can't approve since he is the reporter.