Contrary to what seems to be its popular usage, the ad hominem fallacy isn’t synonymous with “hurling insults”. What it means, literally, is “to the man” and that’s what it means. In other words rather than debating the arguments that an individual is making, you’re dismissing his arguments because he’s making them.
The most common version of this in the blogosphere at least to my eye appears to be what I call the “once wrong, always wrong” presumption. Even if someone has been demonstrably in error on one or many things or is completely reprehensible, it doesn’t relieve you from addressing the arguments that individual is making rather than just dismissing them out of hand.
I wonder what brought this on? 😉
Another common version is to confuse the message and the messenger.