As I see it it's pretty simple: if something fits into ANY of the normally accepted definitions of art, it can not be objectively denied as art.
Wether it's good art or not, is a completely different matter.
For reference, here's a list of some such definitions from wiktionary:
(uncountable) Human effort to imitate, supplement, alter, or counteract the work of nature.
(uncountable) The conscious production or arrangement of sounds, colours, forms, movements, or other elements in a manner that affects the sense of beauty, specifically the production of the beautiful in a graphic or plastic medium.
There is a debate as to whether graffiti is art or vandalism.
(uncountable) Activity intended to make something special.
(uncountable) A re-creation of reality according to the artist's metaphysical value judgements.
(uncountable) The study and the product of these processes.
(uncountable) Aesthetic value.
(uncountable, printing) Artwork.
(countable) A field or category of art, such as painting, sculpture, music, ballet, or literature.
(countable) A nonscientific branch of learning; one of the liberal arts.
(countable) Skill that is attained by study, practice, or observation.
According to some of these, games are art.
This discussion, as I see it, is pretty pointless.