delete a ‘feature’ branch using gitflow without merging into master branch

It seems that I can just delete the branch according to the following sources:
Evan Hahn
dominiksymonowicz

To quote Evan:

To trash a branch using git-flow , simply delete it like you’d
normally delete a Git branch:

git branch -D whatever/branch/you/wanna/delete

Note that this does a
forced delete, so anything you did on that branch will be lost. You
can be safer with the lowercase -d flag instead.

So using the sourcetree GUI interface i first changed to another branch ( in my case develop) and then simply right clicked on the release branch name in the list of branches in the left hand panel and selected Delete release/releaseName. This gave me the option to do so with as a Force Delete, which in my case was necessary as I wanted to delete the unmerged commits as well.