There is a nice and simple tool for that from Stefan Muller: https://stefanmuller.com/firefly-removal/
Unfortunatelly for some reson the original file is not available anymore (0 byte).
So here is the original gizmo: https://www.dropbox.com/s/qg8zh743em6oh5j/FireflyKiller.gizmo?dl=0
You can install it, or you can use it directly in Nuke.
For direct usage:
- Open the gizmo in a text editor
- Rename the Gizmo to Group at line 3
- Copy the content
- Paste in Nuke (just simply press Ctrl+V)