Well, there are a number of things we might have to delve into here.
First of all, Germany (and most European countries for that matter) has something similar to the First Amendment to the US constitution.
In Germany, freedom of speech is covered by Art. 5 Abs. 1 of the so called Grundgesetz (Basic Law for the Federal Republic of Germany).
It is stated that:
(1) Jeder hat das Recht, seine Meinung in Wort, Schrift und Bild frei zu äußern und zu verbreiten […] Eine Zensur findet nicht statt.
Translated, that means:
(1) Every person shall have the right freely to express and disseminate his opinions in speech, writing and pictures, and to inform himself without hindrance from generally accessible sources. Freedom of the press and freedom of reporting by means of broadcasts and films shall be guaranteed. There shall be no censorship.
This is thus rather similar to the way freedom of speech has been defined in the US of A. However, both in the USA, as well as in Germany, it has been decided for a variety of reasons, that there ought to be exceptions to the First Amendment/Art. 5 in certain scenarios in an effort to create a basic ground on what may be unacceptable to be expressed in any context.
In the States, this has largely been one of the many jobs the Supreme Court had to deal with, deciding on limitations to the First Amendment, essentially making it what is known as a "limited right".
A list of the things that are not covered by the First Amendment's guarantee of free speech and expression may be found here: https://en.wikipedia.org/wiki/United_States_free_speech_exceptions
Adding to that, obviously, the First Amendment only applies to Government censorship. Restriction of speech in a private context, either by a TV station not airing specific ads for certain reasons (e.g. youth protection, ...) or any other private entity, are usually of course not protected by the First Amendment. Other provisions like laws attempting to ensure non partisan coverage or anti-discrimination bills may apply here, though are holy separate from the Bill of rights.
Now, I am saying this, to give some context, because similar things do obviously apply in Germany. However, because of the historic, cultural and especially political differences between Germany and the States, certain things that might be excluded from Freedom of Speech protection in the US are "fair game" in Germany and vice versa.
Which bring us to the case that prompted this discussion in the first place. Because similarly to how an US moderator would react if any of the things exempt from the 1st Amendment were posted on his/her forum, we also had and have to react if something like this happens covering one of the things exempt from Art. 5.
Thing is that in Germany this all is a bit more complex. Because while the US have a more or less set list based on Supreme Court cases, the German law states on paragraph 2 of Art. 5:
Diese Rechte finden ihre Schranken in den Vorschriften der allgemeinen Gesetze, den gesetzlichen Bestimmungen zum Schutze der Jugend und in dem Recht der persönlichen Ehre.
These rights shall find their limits in the provisions of general laws, in provisions for the protection of young persons, and in the right to personal honour.
Now, German general laws are rather long which creates a bit of ambiguity in certain scenarios. However, as most of these laws have been created based on the shared values by German citizens, this, while a bit more complex than the US equivalent, is just the same.
In the particular case that started this discussion, specifically the law of § 130 Volksverhetzung.
Now, in the US of A similar statement would have likely been prevented by the First Amendment limitations on "offensive speech" though the situation would have been less clear.
In any case, it is thus not very unlikely that a similar reaction would be appropriate even under US laws.
Last but not least, how should we act. Well, simply put we have to remove it. Replacing it by a "cheeky message" would obviously be a good way as to show that "something" was there at one point in time.
Have a nice day,