All the content on Aether is text — it's pretty hard (but not impossible) to make text illegal. Of course what is on your computer is your responsibility, nothing changes that. There is a SFW list enabled by default that only shows the 'safe' communities to you in the app, but your app still communicates everything as of today.
However, this is actually the most common feature request we have right now, an ability to block certain communities from transmitting. We are converting the SFW list to a 'filter lists' feature, much like adblock filter lists. These lists can be whitelists or blacklists, and they will be able to control not just visibility, but also the receiving and transmittance of content as well.
So the expected behaviour is that if a community is in your blacklist, your computer will never fetch that content from that community by checking against its fingerprint. That should be helpful to solve this issue. We'll be providing a default whitelist as well.