No biggie, all you need to do is sink an enemy ship of equal weight (at least) and your negative renown will be cancelled out by the positive aquired from the new kill.
This is actualy very common, and some gamers sink neutral ships on purpouse out of principle, like when you find "neutral" ships formed up inside enemy convoys, clearly aiding the enemy's war effort
(the Americans in particular, are very pro-allies way before they technically enter the war)