Yes, I think you can also say that Mentem affects all spirits, and posit a spell Ring of Warding Against All Magical Spirits, like @dc444 did with his example magus in this thread:
Both the broader and the narrower definition of spirits affected by Mentem work just fine. My reasoning was that I don't want to have three Forms that work against a spirit (Element, Mentem, Vim), and that IMO Mentem is already the strongest Form, even with the narrower definition.