Marsu requested this sometime ago on the forum and a1ex stated he would like to but didn't know how, well, it's easy. Click on the branches tab right above here, next to filters click on "Merged", on the far right of list items you will see ... (three dots) if you hover, click that and go to "close branch". See the attached screen shot.
I did this to my fork to figure out how and so that I could just try to submit this as a pull request but that won't work, the pull request dialog thinks there's been no changes (which technically there haven't been changes to a particular branch, but a change to the repo itself, which apparently isn't pull-requestable)
Also, in the future please close branches once they have been merged back in if they are no longer going to be used, which really they shouldn't, just create a new one.