Git Flow has the ability to automatically delete the remote branch (along with the local branch) of a feature/hotfix/etc after finishing using the -F option. The -F option fetches the remote branch before it deletes it remotely so you don't lose any work.
git flow feature finish -F branch_name

It would be nice to have the same function available from within SourceTree.

See https://answers.atlassian.com/questions/171850/sourcetree-and-git-flow-remote-branch-delete-on-finish