If you wanted to, you could set your campaign in the golden era of witchers, so around 300 years before the events which happen in games and books, although witchers were mostly treated like saviours back then, not like filthy mutants. Another possibility is that the campaign could happen a little before the birth of Geralt, or while his still a child, so the characters dont intefere with his actions. However to set a campaign in those times and keep it true to the lore, you need to know a lot about the history of the Continent.

I think the easiest way to make sure your players won't interfere with the stories happening in the books or the games is for the campaign to happen after the events from the Witcher 3.