The proper noun Spam itself is used as an adjective; for example a Spam sandwich or a Spam recipe.
The common noun spam can also be used as an adjective; for example a spam overload in my 'in box'.
it is gross, just like spam
Spam itself could be used as an adjective, although other versions like 'spammy' and 'spammish' might be more or less correct.
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
Vietnamese is the proper adjective for Vietnam.
The proper adjective for Georgia is Georgian.
The proper adjective for George is Georgian.
Freudian is the proper adjective for Freud.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.