Starting today, you can lock conversation on an issue or a pull request. If you're a collaborator on a repository, click the lock in the sidebar of an issue page to lock the thread:

This will be reflected in the conversation timeline:

Users who aren't collaborators on the repository won't be able to comment further:

Repository collaborators will still be able to continue the conversation on a locked thread if they'd like.

Remember that, in addition to conversation locking, you can also block or report a user to help keep GitHub a safe community for everyone.