SourceTree doesn't support force push. This is a very common requirement for OSS contributions where people require you to rebase your branch after a code review with a clean history for Pull Requests.

Force push is intentionally not supported https://answers.atlassian.com/questions/54469 because of its hazard of overwriting other peoples changes and I am not asking for that.

There is a safer force push option available --force-with-lease http://git-scm.com/docs/git-push that checks if no one else has pushed after your pull then your rebase is okay to force push. Would be great if this was supported.