Did you buy it as DLC?, MS do not give refunds for that. You could try returning it to the game store if you purchased it recently.
What you should do is contact Rockstar.
Tell them you just bought the game, and that it is not playable as it infested with modders.
So you either want a refund off them, a replacement game or even better they could support the title and do something about all the modding.
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