Track your time and bill for it. Maybe have a 6 month grace period where you send all clients an invoice that's zeroed out with a note about future support rates. Screenshot the incoming phone call and when the call ends...or let all calls go to voicemail and call back after creating a support ticket. Thousands of dollars are given away every year in an effort to get that service call off your plate.
Avid Stamp Collector - I really love 39 Cent Stamps