Actually some letter-combinations are in fact banned:
SS (Schutzstaffel), SA (Sturmabteilung), AH (Adolf Hitler), HH, 88 (Heil Hitler), NSDAP and others. Obviously, youre not going to get charged for scribbling them on some paper, but you won't be able to get a personalized number-plate (for instance) with those abbreviations (on that note, the Brits also ban certain number-plates. usually not in connection with the second WW, though.).