I did not know that ‘Delete your account’ is a long-running meme, but as the LA Times explains here :
“Delete your account” may seem like a funny retort, but it’s got some cultural weight to it. The phrase has been a long-running meme, particularly in the Black Twitter community, and is used as a combination of insult and advice. Sometimes, if someone says something so stupid that it is beyond redemption, people will gently, or harshly, advise them to remove themselves from the Internet.
Or: “Delete your account.”
So when Hillary Clinton responded to Trump’s attack, that was what was behind it. Will Trumps insulting tweets help him or hurt him in the next few months in the run-up to November? We are sure to find out.