I believe it's in the basic.cfg, near the top.
Look for
[RENOWN]
RenownReachGridObjCompleted=500
RenownPatrolGridObjCompleted=200
CompletedPatrol=0
NEUTRAL=-1
ALLIED=1
AXIS=-10
WrongShipSunk=-5000
FirstRankRenown=100
SecondRankRenown=350
EndCampaign=-5000
Change the value for
NEUTRAL. I guess if you changed it to zero you'd neither gain nor lose renown if you sank a neutral vessel. Better wait for an older hand to confirm, though, I've not tinkered with this myself.