For some reason Atlassian SourceTree not only disallows force push by default, but actively hides the feature. I suppose this is to prevent people new to the GIT versioning system from performing destructive operations.
Unfortunately, developers often wrongfully assume SourceTree does not support this feature. Thankfully there is a very simple way to enable force push in the UI.

After going into the application preferences, there is a tiny checkbox squirreled away which enables force push.
Warning
With great power comes great responsibility. Make sure you understand what you’re doing in GIT and never I mean never force push over a branch you do not “own”. Also, if you force push to the master
branch while sharing a repo with others, you probably deserve to be murdered in your sleep.
You have been warned.
Further Reading
How to use GIT reflog to recover lost commits