jointhefediverse.net seems to be a commonly linked resource for directing people to join the Fediverse.
Curiously, it does not list Lemmy under the list of Reddit alternatives. Their GitHub README explains why.
Previous relevant discussion: https://lemmy.ml/post/78808
Searching for the instance and then tapping on the element gives you the menu that provides the option to block on the third party app Mlem.
I use Voyager, but this helped me find it in the filter settings. Thank you!