Removing Frame Markers
You can remove frame markers from the same menu from which you can add frame markers.

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In the Timeline view, select the frame containing the frame marker you want to remove.
TIP If you want to remove several frame markers, select all the frames containing the frame markers you want to remove. -
Right-click on the frame and, in the context menu, select Mark Frame > Remove Frame Marker.
The frame marker will be removed.