Good evening.
First of all, thanks for purchasing this plugin.
To answer your question: under the 'rules' heading in the plugin parameters, you will find 'Block Resistance' and 'Flat Block Value'. Block resistance is a number that the incoming damage is multiplied by when the defender blocks. By default, it is zero, meaning all damage from the incoming attack is blocked. Set this to 1 and all incoming damage will be left alone.
The second value is the flat amount removed from all successful blocks. It is also set to zero by default, but you'd want to change this, likely. You can also leave it at zero and set the <Block Value: X> note tag on your shields to make some shields block better than others.
~Ramza