When version 0.19 arrives, you will be able to block any instance you want (see pinned post in [email protected])
For now you need to block them in the app you are using. If you're mostly using it on the website, unfortunately you will need to manually block the communities you don't like.
Apologies if this isn't what you mean, not quite sure what you're getting at with "mobile users".
But if your mobile app doesn't have blocking functionality yet it should at least have "open in browser" or equivalent wording in a menu somewhere, so you can open the current user profile or community in the web view and block it that way.
Until proper support arrives (Lemmy 0.19 and... whenever it's meant to appear on kbin, which currently even errors trying to block a certain hexbear community), I just made a few filters. Unfortunately they are on my other machine. Fortunately they are plain old uBlock Origin selectors, and not that hard to make. The rough points:
On kbin, posts on the front page, magazine (community) pages etc are article tags, containing links to the user and community. That link will always end with /u/@someone@someplace or /m/somewhere@someplace. As a result you can do stuff like block article.entry:has(a.user-inline[href$="@hexbear.net"]) (posts by all hexbear denizens) article.entry:has(a.magazine-inline[href$="@hexbear.net"]) (posts by anyone to hexbear communities) - this is untested off the top of my dome, syntax might be broken.
I don't know how it works in Lemmy, presumably not that differently.
I had an issue blocking a certain community (on kbin) and was then able to block the domain. I did not really want to block the whole domain but since it was erring on the magazine and I found that one particularly annoying I finally blocked the domain.
Domain block supposedly doesn't work that way. Or at least not properly. I don't remember why - it might be that "domain" refers to the link and not the community, as on Reddit. Instance level blocking is supposed to be the "proper" fix here - when it arrives.