Here's a potential e-mail to craft*;
Sorry, I'm very busy right now with my existing contracts and not
regularly checking my e-mails. If you have a support request for a
product I used to work on, send a message to
"firstname.lastname@example.org", so that we can discuss setting up a
support agreement. If it's urgent, be sure to prefix the subject with
'URGENT:' and I'll move it to the top of my priority support list.
Send this in response to any support request, even if you're sitting in bed eating a sammich and checking your e-mails. If it actually does look like the kind of thing where you'd want to help them immediately because it's a big deal, still send this, but immediately follow up with an e-mail saying "Just saw this, I'll help as soon as I can".
The idea here is to put them in the mindset that this is a service you provide as an extra, and if they want it, it's a chargeable service. The word discuss means that you haven't committed to anything, and by allowing them to specify 'URGENT:', if you've actually put them in a bad position (e.g. some mistake you'd happily fix just to maintain your reputation and because it's the right thing to do), you can approach these with a different mindset. "That website you made for us just lost us $50,000" is something I'd approach very differently to "we need you to change the position of the logo on the 'About Us' page".
This means that if you do make time to help (because being helpful is a good thing), then they're aware that there's been a cost to you. And if you don't, then they're aware of your reasons.
As someone mentioned, if you have an existing support agreement, or if they're under the impression that you do, you'd have to provide service anyway, but this response covers this (and they should have your support number anyway). Also, you should go into any job negotiation with the intention of clearly specifying your support terms up front, especially when it's a solo project and thus you're not working off a standard template contract for all devs, and even more especially as this has come up for you before.
How do you know if you're professionally obliged to do something? Because your professionally drawn up contract obliges you to do it. How do you know that you're not? Because the limits of that obligation are specifically mentioned in your contract (e.g. 'The employee shall provide support on all aspects of the work until the termination of employment by either party, as described in section 3'*).
*: Anyone has any improvements to the wording, please leave them in the comments