Just tried to uninstall your client but when I ran the uninstaller all it uninstalled was the uninstaller. I know that sentence was hard to read but I couldn't figure out a better way to make it understandable.
The game is not installed afaik... when I downloaded it was just a compressed files with the needed files inside.
To uninstall just delete the game folder.
And alternatively the registry keys located at HKEY_CURRENT_USER\SOFTWARE\SonicTeam\PSOBB (can just delete SonicTeam and it will do the job)