As a Bubbles user, you have the ability to delete any bubble you're an admin of. Most commonly, that means a bubble you've authored, or a meeting bubble sent to you by Bubbles Notetaker. Here's how to do it:

From the Activity or My bubbles view, hover on the bubble you wish to delete and click the ••• icon on the far right:

Open the dropdown menu and select Delete bubble at the bottom:

Finally, confirm the decision by clicking the Delete button in the confirmation step:

After that, your bubble will be permanently removed and inaccessible.