Short answer: No way to really disable it because the engine not running a lot of the time is pretty integral to the whole being a hybrid thing.
Longer answer: It doesn't really matter because the vehicle is being put in motion by the ~120 HP electric motor since it's more efficient at low speeds unless it's one of the circumstances where the PCM has the engine still running when stopped (low charge/high load like A/C, HV batt reconditioning cycle, warming catalytic converters, or warming coolant for cabin heat). If at a stop you spike the accelerator with more load than it can meet like because the HV batt is low then it'll start the engine right off as well. But there's also no appreciable lag like on ICE-only start/stop engines where a pinion gear has to move, mesh with the flywheel, and start. The engine is always connected mechanically to the starter/generator motor and it's fairly powerful so starts very quickly.
Longer answer: It doesn't really matter because the vehicle is being put in motion by the ~120 HP electric motor since it's more efficient at low speeds unless it's one of the circumstances where the PCM has the engine still running when stopped (low charge/high load like A/C, HV batt reconditioning cycle, warming catalytic converters, or warming coolant for cabin heat). If at a stop you spike the accelerator with more load than it can meet like because the HV batt is low then it'll start the engine right off as well. But there's also no appreciable lag like on ICE-only start/stop engines where a pinion gear has to move, mesh with the flywheel, and start. The engine is always connected mechanically to the starter/generator motor and it's fairly powerful so starts very quickly.