there is no need to overthink it and come with complex community and voting approaches, the described use-case is simple, person wants to own a group and only be the one that can change stuff and invite people to the group that are guests/fans, troll can’t never troll because they can’t edit the group state at all
the problem about someone sending spam in the group is another topic, it is almost impossible in Delta Chat to ban people from a group because they can always join again and send more spam, another tooling could be added around the “owner” approach like owner saying “no messages with links should be allowed”, “no media allowed”, “no more than 1 message per minute allowed”, “ignore messages containing THIS word”, etc. so this way they can limit spammers
and ofc one simple measure that can be added even before the more advanced features mentioned above is that “owner” can delete messages for everyone, so even if troll/spammer sends stuff the owner can delete everything from that person and with low effort undo for everyone the damage of the spammer that then gets less motivated, this mixed with the bot approach gets you already a bot that can auto-delete messages containing links, media or banned words, it can even auto-kick/ban members based on post etc
so to summarize:
adding group creator/“owner” feature + “owner” can delete messages from everyone + bot, would get Delta Chat already pretty usable for public groups