You'll have to look for the 'block community' button. Depending on the UI you're using, this can be in different places.

In Lemmy's default UI, you first click the part under the title identifying the community under which it was posted. Let's say it was posted under darkmemes@lemmy.world. What you do is:

1. click that darkmemes@lemmy.world;
2. on the community page, you then click the 'block community' button on the sidebar.

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On a phone app, the process may be less cumbersome. On Thunder, an Android app, you just have to click and hold the post, wait until a menu appears, and then select 'block community'.