According to New York Magazine's Daily Intel, Avoidr was created by San Francisco–based technologist Jesper Andersen after seeing a bunch of his friends go through breakups.
Basically, the app works as a plugin for your Foursquare account (much like apps like Assisted Serendipity, but even more simple). You can label your Foursquare friends with a variety of colorful terms, and then choose to hit "Avoid." Clicking on "Places to Avoid" will yield a list of venues at which your foes are currently present. Unfortunately, it appears as though this is only a web-based app. It would be much more useful if it e-mailed or pinged you alerts, like the aforementioned Assisted Serendipity.
Check out what it looks like below. I added Mashable Editor-in-Chief Adam Ostrow to my Avoid list -- which is totally a joke, everyone. I'm sitting three feet away from him.
This isn't the first app that's acted as a kind of foil to social media -- Please Rob Me functioned in a similar way, allowing one to scan a plethora of user checkins. And then there was Seppukoo, which allowed one to kill his Facebook profile.
Still, Andersen isn't against Foursquare per se -- just the awkward situations social networks present (i.e. unfriending someone is kind of a slap in the face). So if you've just split with a tech-savvy beau, you might want to check out Avoidr, so as to avoid a run-in via checkin.