turn on boiler just to heat tank but rads heat as well?

Hi , now its Spring I dont need the radiators to come on but need to turn boiler on occasionally to heat tank for general hot water. Trouble is the rads in the spare room and bathroom heat up as well, all other rads stay off as they should. I turn the hreating control to the rads to 0 and click the switch to hot water only, any ideas please?