Yea I think the approach you have decided to use is simply the best and trust ne it works wonders. Simply ignoring them would tell them that they are busy wasting their time and in no time, they would learn to do the right thing. When you keep flagging them, they enjoy the attention you give to them and so continue.
Talking about steem invite, I have never used that app and don't even know how to go about it. Would try to give it a try as onboardingnhas been quite difficult for me lately . It doesn't cost much so why not.
Those who have a large fan base in other platforms should really try to help populate this platform by advertising steem all around. Thinking of starting up a steemit campaign on my Facebook page which I would get to post updates here on steemit.