I use this in my post report extension but you can do it as Xal0lex mentioned with some other tools that inject scripts. You can probably just run it in the dev console (Ctrl-Shift-I) manually. Not sure about mobile browsers though.
try {
document.getElementById("smfheader").style.visibility = "hidden";
document.getElementById("hellomember").style.visibility = "hidden";
document.getElementsByClassName("avatar")[0].style.visibility = "hidden";
} catch { }
Looks like this:

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