If you want to leave only the <font> tag, then you could use a regular expression to replace the whole <font...> tag, as in...
var regexp:RegExp = /<font.*?>/;
myString = myString.replace(regexp, "<font>");
But if you want to specifically remove only the face portion of it, then you could use...
var regexp:RegExp = / face.*'/;
myString = myString.replace(regexp, "");
I was going to add that have a <font> tag that specifies nothing is about as good as not having one at all.