Next time you talk to a customer that strikes you as incredibly average, somebody so nondescript that you realize are having a hard time remembering their features a minute after you get done talking to them find a reason to say something else to them so you can fix them in your mind.
Make a big cut out poster of them in your memory, like the ones of Johnny Depp as Captain Jack.
Then whenever you see your ex put that customer over them.
The initial effort of putting the cut out over your ex will be tough be do it a couple of times and you will find that they just slip out of your mind just like the customer did.