If you've got something to say,

then pick up the guts and say it to that person's face. Don't hide like a little chicken shit behind the anonymity of the internet, yet talk trash and run your mouth like nobody's business. If you got something to say, step up and say it out loud and deal with whatever comes back at you. 

And if you don't have the guts to say it straight out, then shut up. If you have nothing nice to say, just shut up. If you feel that your mouth is just so itchy and you cannot handle shutting up, then have the guts to say it straight out.