Have you ever experimented with GitHub Pages or some other "deployments" in your repo only to change course later? Have you ever furiously searched for a way to remove old deployments from your repo's meticulously crafted page?
...yeah, me too. =/
GitHub doesn't expose a UI for this, at least at the time of writing. The only supported way of removing the "Deployments" widget from your repo is by removing all deployments through the API.