Is that what you are REALLY trying to do, or are you instead trying to prevent a caster from detecting certain specific magical auras on certain things or people within that area?
I think a Nondetect spell is what you are after, in either basic or its Communal form, that is triggered when detect magic is cast. Using a spell trigger gets around probable spell duration issues, as Widen Spell won't work on Nondetect and ruling that Permanency may apply to Nondetect may lead to pernicious results you may regret. It allows for a Will save, but there it is; Detection spells should ordinarily work, after all.