Most Helpful Guy
Whatever you do don't be mean back and don't try to sabotage him, if anything this will result in making you look like a "Bitter Ex" and he could pull that card (She's just trying to get me in trouble). What you need to do with dickheads like this cunt is outperform him in every way, shape and form. Just take your work more seriously, brown nose to the executives and put your nose to the grindstone, that will get your work ethic noticed and when they start noticing you they'll start noticing him being a cunt, and then because you've become a valued employee, he's going to be on thin ice. Trust me, I've pulled both sides, and being vindictive about it usually blows up on yourself, you gotta beat him most times and get the upper hand.1
Most Helpful Girl
This is definitely something you need to bring up with HR.
This is workplace (obviously) harassment.
If they're not taking this situation seriously, even after bringing it up to his supervisor, simply because they need him, that's something that can get the company in a lot of legal trouble if they continue to ignore your reports on this ex.
You need to report it to his supervisor again, or to HR or upper management and "threaten" them by saying that if they continue to ignore this situation, you'll take legal action; whether you really do or not is up to you, but you need to let them know that 1) you're no pushover when it comes to this kind of harassment, and 2) that the company can be in legal trouble if they continue to push this matter aside.1