This is a pain we feel as well. Once you help out a customer it is too easy for the customer to directly contact the developer later on and request support. And since we usually aim to please, and probably feel sort of responsible when the application we built for them has a problem, we too often give the customer a quick helping hand.
I think that the developers should be separated from the customers, but this requires that the company has a support/concultancy department which can fix the problem instead. They in turn should be free to contact the developer, unless it's a huge company with a mainstream application where there is a less risk that the problem can be traced back to a problem with the sourcecode.
But let me tell you, I understand how difficult this is. I've been working in our consultancy shop for many years, starting from support and now I'm mostly managing the other consultants and developing. There are a lot of customers (like hundreds) who feel they have a personal relationship with me, and assume that they can call me directly even after years and years.
My tip is to make sure you have a good network of concultants and supportworkers who can help the customer for you, and have them contact you instead if they can't figure it out.