If you found the variable in the game files, can't you make the value 0? Or just delete the line that says it deals X block damage.
Found these lines in my items file for the AK for instance:
<passive_effect name="BlockDamage" operation="perc_add" value="-.15,.15" tags="perkMachineGunner"/> <!-- random BlockDmg -->
<passive_effect name="BlockDamage" operation="perc_add" value=".1,.5" tier="2,6" tags="perkMachineGunner"/> <!-- tier bonus -->
I'd either set the values to 0 so:
<passive_effect name="BlockDamage" operation="perc_add" value="0,0" tags="perkMachineGunner"/> <!-- random BlockDmg -->
<passive_effect name="BlockDamage" operation="perc_add" value="0,0" tier="2,6" tags="perkMachineGunner"/> <!-- tier bonus -->
Or just delete the whole line or close it in ignore brackets <!--IGNORE CODE IN HERE-->
If neither of these works, check if the bullets themselves carry any block damage. Not sure then. But good luck, hopefully someone has tried something like this before.