Ah. I see. So a bit different. It is, "You know that file I told you to include in the next commit? Forget about it."
So bk uses a different model. The revision changes to files are tracked in 'sfiles' for each 'gfile'. When you say
bk new FILE it doesn't mean, "When I make a commit next, include this file". It actually creates a revision history file for FILE and leaves a "pending delta" for that file. So to revert that change you want to actually remove that pending delta.
So rather than
bk rm I used above you want to use
bk fix FILE instead. That will strip the pending deltas from FILE, but keep the sfile. It is very similar to
bk stripdel -r.. FILE, but it doesn't delete the user file (the gfile).