Dont ignore it. Even if they are 100% wrong, if you dont show up to the trial, they win. With the win, they could attempt to get a judgment and place a lien on your house.
Chime. This thread is mistitled. It should be, "Can former owner sue you after closing if you close late and the delay costs them money?" Yes, they can (and they have). If you don't show up and they do, they win and not only does that mean they can put a lien on your house, it will also show up on your credit report. Not a good thing. But did they sue you in your own state/city, or in the town where they are now? If in your state/city, are they really going to fly back to show up for the hearing? (BTW, call the courthouse where you were sued and ask what the procedure is for small claims--is there a hearing? There almost has to be, but you should still get the lay of the land.) If they sued in the state/city where they are now, and you have no connection to that place, you may be able to get the case dismissed for lack of jurisdiction.
I would talk to my real estate agent, then get a copy of the complaint (or whatever it is that they filed in small claims court) and go talk to a lawyer. But the easiest way to resolve this is probably to offer them, say, $500 to drop the suit. You could try that and if they say no, then use that $500 to go talk to a lawyer (actually it shouldn't cost that much--you probably need less than an hour of the lawyer's time to get a good idea of what you need to do). BTW, you need a lawyer in the place where you were sued who specializes in real estate litigation.