If you're using Sync, it's the three dots on any post or comment, then Filter. You can choose to filter (block) the user, the community, the linked website, or the entire instance hosting it.
Three dots usually indicate "options" or "settings" when using Android. I'd invite you to tap those buttons any time you see them and learn more about the tools you are using.
For Sync, "filter" and "block" are separate functions, blocking a user or community will also block them on the site you use, but filtering them only removes the posts locally on Sync.