Every time I go to a Jewish event in Philly, I run into guys I've been out with on bad dates. I'm ready to stop going to events because I'm sick of these awkward situations. Is there anything I can do to avoid these people or to make it less awful when I see them?
Do yourself a favor and just decide that it's not that awful! Sure, you're going to see these guys, but that doesn't have to turn you into a hermit or a misanthrope. A smile and a wave followed by an utter lack of interest will go a long way in discouraging a drawn out encounter. Confidence and poise will also carry you through even the most awkward situation.
If you see a guy across the room who you once dated, you don't have to go over to him or even acknowledge him. Just try to have a good time by talking to new people and doing your own thing. If he comes over to you, say, "Hi, nice to see you," (even if it's not) and go back to your conversation or to the restroom or to the bar. Don't say, "How are you?" which will elicit an answer. Don't get stuck staring at him or forcing yourself to try to make small talk. Though I normally tell people that showing up to Jewish events alone is totally fine, if you feel like the security of a wingwoman or man will make these experiences more tolerable, bring a friend and make sure s/he has your back any time one of these reruns comes over. Or, use his approaching you as an excuse to turn to someone new and introduce yourself. Instant best friends can be made by bonding over bad dates!
You can choose to be more explicitly rude. "Please don't talk to me," will typically stop even the most brazen guy in his tracks. Regardless of how bad the date with him was, though, you end up looking like the jerk if this is how you treat him. You presumably want the opportunity to date other people in this community and you don't want a great guy to overhear you telling someone off. However, if someone is truly harassing you, feel free to be as assertive as you need to be (and tell the organizers of the event!).
Of course, if we're talking about an actual ex-boyfriend or someone who you had hoped would call and didn't, I understand that becomes much more difficult. I still think the best path is looking great, acting confident and not dwelling on the potential for awkwardness. Don't use the event as a forum for trying to find out what went wrong or who he's seeing now or what he thinks of you. None of those mind-consuming activities are worth your energy.
In summary (consider printing out this paragraph and stashing it in your purse!), keep going to events. Bring friends with you. Show up looking and feeling your best. Don't drink so much that you'll be tempted into bad behavior. Be friendly but firm. Surround yourself with new and interesting people. Don't dwell on the past or share how awkward you feel with everyone you talk to. Imagine the best case scenario considering who else is in the room and make that happen for yourself. If nothing else, know that you're not alone in trying to avoid certain people, so find an ally with the same goal and find ways to laugh about it. You'll be fine.