[silicon212]

It's called "dieseling" or run-on. It happens for a number of reasons, some of the more common are vacuum leaks, hot spots in the combustion chamber due to carbon deposits, or high idle speed.
Sometimes, over-advanced ignition timing can lead to detonation (that rattling, pinging sound you hear under acceleration sometimes), which leads to hot spots in the combustion chambers.

Make sure all of your vacuum hoses are in good condition with no cracks or breaks. Make sure the engine idles in the 800-900 RPM range when the transmission is in park or neutral. Make sure the timing is set to factory specifications as listed on the emissions label affixed to your radiator fan shroud. Generally, this will be 8* BTC with the 4-wire computer interface disconnected from the distributor and the engine idling and at operating temperature (you WILL see a Service Engine Soon light during this, ignore). If the engine continues to diesel after doing this, there are safe and not as safe methods to decarbonize the combustion chambers. The safer way is to use GM's upper cylinder cleaner (follow directions on the can), and the not-so-safe way is to slowly pour water (1 cup should be all you need) into the carburetor air intake (the front, primary side - where the choke is). You want to do this again at operating temperature and with the engine maintaining at least 2,000 RPM as you slowly pour water into the carb. Failure to follow this exactly can lead to a hydrolock which can destroy the engine (spun bearings, bent connecting rods, blown head gaskets), which is why it's the not-so-safe method. After doing this, leave the engine running, put the air cleaner back on and drive to the nearest freeway. Clock that thing wide open up to freeway speed, then take your foot off the pedal and let the speed drop to about 40 or so (you obviously don't want to do this on a crowded freeway), then hammer it back to the speed limit - repeat this cycle about 5-10 times to dislodge all carbon not taken out initiallly by the water.

This should fix it - if not, then you might need to rebuild the carb itself.