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.
If you need to simulate riding steeper hills than you can train on, you can just use a cassette that doesn't have the lower gears.
For example, if your bicycle has an 11-28 cassette, replace it with an 11-25.
- take rear wheel off (which you should know how to do because you have to be able to address a flat tire while riding)
- unscrew lock ring
- remove cassette
Literally - that's all that removing a cassette consists of. Given you'll have to swap the cassette back for the actual cyclosportive itself, you really don't want to pay for that work.
You will want to have someplace to put the dirty cassette when you're not using it - a plastic freezer-type bag works well. You can even label it with the cassette specs (for example "10-speed, 11-28"). Also have a relatively long plastic zip tie handy when you remove the cassette so you can immediately zip-tie it together in the proper order before you put it in the plastic bag.