Open the Aerosoft Launcher that was installed with the game.
Then, go into the "Simulation" category. (It's at the bottom when you open the launcher I think).
Click "OMSI - Der Omnibussimulator"
Click "Uninstall".
Follow the Instructions.
P.S. The Uninstall instructions are in German, so you may want to have a program like Babylon or Google Translate open.