hg commit --close-branch
should be enough to mark a branch close. (see hg commit)
--close-branch
  mark a branch as closed, hiding it from the branch list.
See also this thread:
  My expectation is that I close a branch because this line of development has come to a dead end, and I don't want to be bothered with it any more.
  Therefore, when a branch has been closed I shouldn't see it (in branches, heads, log, for instance) unless I explicitly ask to see closed branches.
  
  I should note that I expect a closed branch to remain in the repository; 
  it may be useful in the future, and the commit --close-branch message 
  should at least explain why the branch was closed.
  Pruning branches is another thing altogether.
Note: that "closing branch" business is one aspect seen as missing in Git, when compared to Mercurial:
  Branches in git are, we’re always told, ephemeral things to be used and thrown away, and so far as I know git doesn’t have a way to indicate to your colleagues that you’re done with a branch;
  the only way to do this is to delete it, or to hope they see the final merge commit and understand that the branch is closed to further development.
  
  [In Mercurial] When you’re done with a branch, however, you cannot delete it from the repository; instead, you issue a commit which closes the branch, and Mercurial notes that the branch is closed. It’ll remain a permanent part of your repository history.