On my Roku, I subscribe to Netflix, Prime Video, and Paramount+ and don't see ads on any of them. (I don't use The Roku Channel for any of these - I use apps on the Roku home page.) If the same stuff shows up with ads if watched via The Roku Channel, then consider not watching them that way???
On the other hand, maybe you're subscribing to some service with a "hybrid" business model which has both fees and ads (sometimes called: "limited ads" 😀). If you don't like the ads on those, then changing services or changing to a higher tier that makes the ads go away me be an option to consider.
In the end, there are many many apps on Roku, and it's possible to watch with no ads, and it's possible to not pay and watch many ads, and various levels in between those. And there are some apps, which (currently?) have no ads or payments. There are also a couple of library-run apps such as hoopla which I believe lack both ads and payments. So, browse around - there are many options.