Then have an intermission whilst the credits roll. Serve ice cream and refreshments. Make it part of the experience. It'll be fun.
Or sell tickets separaly for the pre-feature and the main feature (or just publish times when each will start and have an intermission in between so if you want to just see the main feature you can without disrupting anyone who arrived early for the pre-featured).
You have no idea who has seen these films and who hasn't. Yes, sometimes I want to go and see an old film at the cinema because I never got a chance to see it there the first time around (Star Wars was a case in point back in 1997). But sometimes I just haven't seen it so I want to see it for the first time, unmolested by spoilers.
There are better and more creative ways that aren't a great deal of effort to implement to handle this than showing a bunch of spoilers before the film you're there to see.