Hey @AquaL1te – It looks like someone from the Discourse team suggested this as a feature back in 2016 (Limit group messaging and @mentioning by trust level or group(s)). At this time, I don’t think there has been any update on limiting group messaging by the trust level. I would say to submit this as another feature, referencing to the older feature request from 2016, to see if there is a possibility to add this to the Interactions tab in Group Settings. They may have the ability to create mockups now and implement this feature.
In the meantime, you may need to continue watching for the spam posts or limit the message to ‘Only group members, moderators, and admins’.