It is sort of in the next update. Users can “block” instances, what this means is that they will not see posts or communities from the blocked instance, however they will still see the blocked instances users posts on communities on other instances, as well as their comments on posts on other instances. This feature should work to block the spam you’re talking about. We haven’t upgraded to that new version on sh.itjust.works yet because it introduced a bug in the federation system that hasn’t been fixed yet.