How to reset the git-flow configuration in SourceTree?

No, it is not possible (at least not at SourceTree 1.4.1.0).

It is possible however to do it manually by editing the file /.git/config in your git repository.

The Git-Flow extension adds it s configuration in the following two sections, which can be freely edited.

[gitflow "branch"]
    master = master
    develop = development/current
[gitflow "prefix"]
    feature = feature/
    release = release/
    hotfix = hotfix/
    versiontag = release/
    support = support/

Resource: http://www.crisi.ch/change-git-flow-settings-in-sourcetree/

In SourceTree 2.x it is easy to edit your GitFlow.

1. Click Settings if you're using Mac (Repository - Repository Settings in Windows)
2. Select Advanced Tab
3. Click Edit Config File ...