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.
How this came about: I made a release branch but didn't need it. Git-flow doesn't let you have more than one release branch out at a time. I thought that there might be some issue with residual git-flow files laying around, but (force) deleting the branch was actually safe.