This is a very good idea, downvoting could be what you have just suggested but flag can be used to deal with these spammers who are just there to get on people's nerves and have not realised what this platform is all about yet.
Furthermore, I think something like an orientation can be carried out to further educate steemians who still do not know the difference between downvoting and flagging just as @aggroed has put down.
Thank you @timcliff