Boiler heating extra zones
Assuming the thermostats are working, you may be getting residual heat once the thermostat turns off.

Do the pumps have speed controls? Many pumps have LOW, MEDIUM, and HIGH settings. A lower setting may avoid spikes in temperature.

Also, many pumps have mixing valves near them that control the temperature of the water being sent to the radiators in that zone. If you have selective zones that are too hot, you may be able to temper (in this case, lower) the water going to that zone.