It should definitely be possible for admins/mods to create a community-level block list. Beyond that, spam is hard problem because spammers can easily generate a new identity for every single note they post. The spam issue is a bit harder to deal with for communities because they don't even have to have anyone following them for their note to be seen. I suspect that the best solution to this problem involves an options where users must be explicitly pre-approved to post in communities. Currently working out the technical details of that.