I’ve just found out that someone committed some time ago some changes locally, that was revision #410, now the right version is #638… that created a branch in that person’s mercurial history, and I want to get rid of it… I tried to execute an Undo > Backout… but it won’t let me…
The error message I got is:

That person’s Mercurial history:

What can I do to get rid of that branch? The changes that it was supposed to do were already made some revisions ago…
That error appears to be because you are on a different branch then the following will probably work:
However, this will add an irrelevant changeset and a merge that you don’t care about. You could simply do this, instead:
The above commands are Unix/Linux commands and it appears you might be on Windows. If you are, just replace the commands with the Windows equivalent. If you don’t understand, I can edit my answer.