Using the command line, I can
git remote prune origin
to bring the list of remote branches up to date with the remote. I cannot find any way to do this in SourceTree. The branches have already been deleted from BitBucket by another user of the repo, but I have the legacy refs on my local repository.
If I right click on each one I can delete it (thought it warns that it will be deleting it from the remote) and that's not what I'm looking for, though it may achieve the same result in the end ...