What it is:
The ad hominem fallacy takes the form of overly attacking someone or casting doubt on their argument based on personal attributes that have nothing to do with the quality of information given.
The purpose is to undermine an argument without having to actually engage with it.
Typically, this is a sign that they aren't interested in having a real dialogue.
How to address it:
Point out that they are name calling and you welcome a response to the actual content of your comments.
"Why should I believe anything you say? You're overweight." (Yes, I've seen someone say this on Twitter. Not cool.)
"Sally clearly has poor judgment and isn't qualified to teach ethics -- she's divorced and sometimes goes too long without washing her hair. "