If it comes on sraight away again then try turning the heater down until it doesn't come on when you stick it in the water.
I believe if you remove the heater from the water and place it back in after some time, it will likely turn on straight away anyway as the thermostat hasn't properly adjusted to the temperature of the water is at the ambient room temperature.
Whats the temperature of your room during the day? I mean if the room temperature is above your set temperature on the tank then thats your problem. The only way you could possibly reduce that is by introducing some sort of cooling, Removing the lid and directing a fan at the tank, hopefully you would "Strip" the heat away. Like when it rains... you're not cold, however when its windy and then rains the wind strips the heat away from the moisture. Hopefully using that might help!
Though if you've got fish in there you really would want to cover it up to prevent them jumping out with something. A thin table cloth could help, white!
So yeah just take some precautions I think, there isn't much you can do though if the room temperature is above your tank temperature unless you introduce some sort of cooling. You can always put a few white sheets of paper in the window to reflect the light out and that really will help prevent the greenhouse effect in your room.
Edit: I'm having this problem with rabbits! Both my rabbits live in a massive hutch in my conservatory however as the sun rises in the morning before anyone is awake it sometimes super heats the conservatory. We did have a timer with a fan so it would set off at like 5am to start cooling however the rabbit chewed through the cable haha. Initially I put white bedsheets over the entrance of the hutch but it only helped keep the temperature down for a bit initially then it would rise up just like the rest of the room.
I ended up leaving the kitchen door open to let them run into the kitchen where it is usually really cool. So they have an entire conservatory and kitchen during the night to live in haha, they got more freedom then the dog sometimes!