Here is the story (no pun intended):

1. The Product Owner had a fairly common feature prioritized in the second sprint.
2. It was estimated with low story points.
3. The story was completed and in testing the team discovered that it didn't work as required for one class of users.
4. The developer who did the work thought it was a permissions issue and should be fairly easy to fix.
5. We logged a bug and prioritized it in the third sprint. It appeared to be a problem with the open source framework we are using. Developer tried installing some patches that appeared could be related. Didn't help.
6. The Product Owner agreed to carry the bug to the fourth sprint at the request of the developer, however with a low priority. On seeing more time being wasted on it, the Product Owner moved it to the backlog.
7. Now we are in the sixth and final sprint and getting the product ready for beta release. The developer, who is highly skilled and very committed, reported that he intends spending own time looking for a solution. Looks like it has become a prestige issue for him.

As the Scrum Master, I told the developer to stop working on anything not prioritized by the Product Owner regardless whether own time or not.