We could choose not to display certain memos for sure, but currently don't. Btw, currently about 10% of memos are not done through the site. E.g. the r/BTC bot.
And someone would make a way to see them in another app. I feel like they will try one day.
I hope someone ends up making one. It'd be pretty nice to have a sleek client that works on Windows Mac and Linux with iOS and Android versions that aren't centrally controlled.