How can I stop the boiler from heating the radiators when only hot water is wanted?
It's extremely difficult to answer you questions accurately without examining the installation. It could be as simple as replacing a defective solenoid valve, or even just correcting the wiring. If your system is actually a single zone setup, it would still be possible to modify it to separate the domestic water from the radiators by installing some solenoid valves and some basic control wiring so the valves open and close appropriately based on the various operating conditions.

Heating your house in the summer every time you need hot water out of a tap is very inefficient. It's worth getting this sorted, a correctly operating system should pay for the modifications in a few years of saved energy costs.