He is not respecting you, so you can be quite blunt.
I recommend writing something along the lines of:
Hey, I feel quite disrespected being ignored. I told you my decision, multiple times, and it's final. I'd like to be there, but I've got other plans, please respect that. I'm going to ignore that topic now if you bring it up again.
or in German (freely translated):
Hi, ich fühle mich von dir ignoriert. Ich hab dir genau gesagt, dass ich nicht kann, und dass sich das nicht ändert. Ich hab an dem Abend bereits etwas vor, bitte akzeptier das. Falls du das Thema jetzt wieder ansprichst werd ich dich einfach ignorieren, sorry.
If he now writes back, just ignore him. Don't ignore him else, but if he speaks about that topic, don't respond. You've made your stance clear, he can't just say "I now assume you will be there..." from now on. If he does, it's not your fault, and you might want to rethink if he really is a "friend".