For most problems with Steem we have to generally self police with the vague and blind faith that somebody in power will one day fix it. But until that auspicious day, manual methods need to be maximised, like my own chrome/firefox plugin that displays everybody we at steemSTEM have blacklisted with an angry red banner.
Memos shouldn't need much effort really. Even something like, accounts with a reputation below 25 get a limited number of memo posts - then leave the crowd/steemcleaners etc to manually flag spam bots.
I dunno. Even FB and twitter have super sophisticated, turing-testing bots by the millions.
There's a limit to what self-governance can do, we need more/better tools to combat spam. Good suggestion.