What do you mean by "delete"? Only removing the branch tag? That would be doable with the <samp>tag cancel</samp> subcommand.

Or do you want to remove all the checkins of that branch from the repository? This would be more difficult, was Fossil try hard never to forget anything already committed. Furthermore there would be the problem, that removed branches could come back by syncing with another repository that still has said branch. So this feature ought to have a very complex semantik I fear ...