Force a push with SourceTree
SourceTree is finally going to add force push: https://jira.atlassian.com/browse/SRCTREE-1156
Reason: https://answers.atlassian.com/questions/54469/how-do-i-perform-a-forced-push-push-f-from-sourcetree
Edit: It is now added in SourceTree, but you need to turn the option on in the settings.
You can show it on Preferences -> Advanced -> Allow force push
Then force push option would be available.
In SourceTree 3 you can allow it in the advanced tab: