Apart from. Stopping her pocket money until the extra I'd paid off I would also reprogram the thermostat to whatever your normal nighttime temp is, evey hour if necessary, so that if she is still switching it on it will at least turn off again when the next hour preprogrammed slot comes around, and so on and so on.
I had this a while ago with my 2 teens; they weren't deliberately leaving it on all night, but as it was set to go off at 10pm, if they were still up they'd put it back on and then forget to turn it off before they went to bed.
To get around this and the late times they were sometimes staying up, I simply reprogrammed the thermostat so instead of going off to 16° at 10pm and then back on to 20° at 6am, I set it at 16° at 10pm, 11pm, 12am, 1am, 2am, 3am and 4am.
Now they're more settled back into a routine, etc, I've changed it to 10pm,12am, 2am as they're always asleep way before then, except for the odd Friday or Saturday night.
Mine was more from a wasteful point of view rather than a money issue as it wasn't regular enough to be causing a big rise in costs.