Thanks to this great topic, I've found the FrontEndToken that I was looking for:
"DeleteGeneratedCells" (well, it is also written in Help but I've found it later :)
Question:
I would love to execute it in NotebookObject without confirmation through the automatically displayed ChoiceDialog["Do You really want to..."].
Is it maybe a case of FrontEndTokenExecute parameters?
I suspect I can deal with it by setting some global options for Mathematica but I would rather leave them.