when destroying wood without any tools.

Changing a vanilla harvest level depends on whether you want to change the level of a tool or a block. A block is easier, as you just have to override the Forge methods that set the harvest level after mods have loaded using Reflection.initialize(ForgeHooks.class);,  (Thanks to diesieben07 for that tip ) then just use setHarvestLevel on the block you want to change.

 

Changing a tool's harvest level is harder, it involves removing the recipe for said tool and creating your own with a different EnumToolMaterial.

Thanks a lot Zer0HD2!!

 

I did like this

        	Reflection.initialize(ForgeHooks.class);
        	Reflection.initialize(Material.class);

        	Material.wood.setRequiresTool();
        	
        	Blocks.log.setHarvestLevel("axe", 0);

Diesieben's method works for all blocks.

I use both to achieve what I want.