One thing to try is to go to the installation directory of the game and run the file called "Unregister.exe".
By default this directory is C:\Program Files\Steam\steamapps\common\ship simulator extremes\ (for Steam) or
C:\Program Files\Vstep\ShipSimExtremes\ (for non-Steam).
This application does not show up, but runs in the background for a split second and cleans some installation stuff up, related to Ship Simulator Extremes.
Hope this helps.