the normal way is to revert the changeset or parts of the changeset and then mark them.
The only risk here is someone re-adding it and this is probably not a big deal as it probably is fine.  But if you want to make sure someone doesn't re-add them you could just add a new demolished building and make a note of the changeset and perhaps old way number in the note so that it refers to it.  Up to you on how you want to go but it's probably fine as it is and this note can be closed.