Disney Channel in the 90s didn't have any ads. And they would show whole Disney movies uninterrupted by anything. For this reason it was a paid add-on to your cable package though, like HBO -- never included in the basic cable package.

In the '00s they still had no real ads, only promo spots for mostly other Disney shows on the channel, and the occasional tie-in with some other Disney property. I think today they have some normal ads but I'm not sure.

They do (did?) exist. Nickelodeon was originally a completely ad-free channel. HBO and Cinemax also didn't have ads.

C-SPAN