this was a problem in version 2.6. I think you have this version installed. We have corected this problem in version 2.6.1.
The problem ist that a background process named pdf24.exe is still running. The uninstaller will not remove the program unless all components are closed. To close this background process right-click the pdf24 tray icon and choose the close entry there. Than you can uninstall PDF24 Creator. You can also use your task manager to close pdf24.exe.
The background process is needed to handle print jobs and therefore to create PDF files if you print something on pdf24 printer.