Heat usually does the trick.
Here's another method in case of future need:
If you plan to have the frame repainted and don't want the seatpin, fill the seat tube with caustic soda solution, replacing the solution every 24 hours or so. Within a week the seat tube will have dissolved in the caustic solution. It will also dissolve paint but doesn't affect the steel frame. This is a slow but good method as there's no risk of damaging a precious frame. You'll need to block up the lower end of the seat tube first. Best not to try this method with an alloy frame though...
1996 TJ Flying Gate, 1987 Rotrax, 1969 curly Hetchins Magnum Bonum, 1956 Bates BAR, 1955 Gillott, 1955 Claud Butler Avant Coureur Special, 1953 Ernie Clements Special Course, 1950 Granby, 1948 FH Grubb lugless, 1938 WJ Hood Palace Special