[1.7.10] How to change so dirt and sand doesn't drop without using a tool

[Erfurt]

Hey guys, so I'm trying to make it so you can't get dirt, sand and other blocks if you aren't using the proper tools. At the moment I have no idea how to do this, any help would be great.

[Failender]

use HarvestDrops event, if the harvesting player has no tool equipped clear the drops list

[Erfurt]

use HarvestDrops event, if the harvesting player has no tool equipped clear the drops list

Okay, how would I go around giving for example dirt a 25% chance to drop, and a chance to drop more than one dirt?

 

EDIT: I should specify, by more than one I mean for example 1 or 2 dirt

[Failender]

Math.random