I'm doing this:

combine diamondhoe
@OverRide
diamond_hoe:*:2
= grass

But it doesn't work, it gives 1 diamond hoe as recipe.

Thanks.

Repairing recipes is one of the many special recipes in Minecraft that acts as a multitude of recipes, but is actually only one recipe. You can disable repairing as a whole, not individual items. If you wanted to disable all repairing. you can set special-recipes.repair to false in config.yml.