Preventing use of low gears while training I'd like to avoid using my two lowest gears while training for a hard and hilly cyclosportive. I am able to resist, but nevertheless do occasionally make a mistake. Is there anything I can do to safely physically prevent myself from using those two low cogs?
You can probably turn the limit screw far enough in to prevent usage of at least the largest cog. Maybe even the second largest cog.

You could also stay on the larger chainring. Depending on your specific cassette and chainrings it’s roughly equal to not using your three largest cogs.

All that being said, in the end cycling is about power output. Using hard gears doesn’t automatically make you faster. Power is force (or more precisely torque) multiplied with rotational speed. Easier gears may allow you to pedal faster, resulting in the same power output over – possibly – a longer duration.