On default hotkey setup (not sure about Lix-like), you press E during a level.
YES, we need this (the editor itself I mean; the fix to the bug is not super critical as, while the resulting text is misleading, no actual functionality is broken). Apologies for the strong language, but seriously, fuck having to save a replay, exit out of NeoLemmix, modify it by hand, go back into the level and load the replay all just to delete an action without erasing everything that comes afterwards.