I work using pull requests and sometimes they need to be reverted. The command line workflow I use is as documented here: http://tildedave.com/2012/11/24/reverting-a-github-pull-request.html
Basically I create a new branch, find the merge commit SHA and then run
In sourcetree there is the 'Reverse commit...' option, which does a git revert but it will not work on a merge commit because it will not use '-m' (see https://answers.atlassian.com/questions/322559/what-exactly-is-reverse-commit).
How can I use sourcetree to revert a merge commit?
If you want to do it in SourceTree, you can only do it with a custom action.
I already created a feature request for this, you can vote for it here.