Uninstall QuickTime player

To clear the list that may appear in the Dock:

1. Launch the application
2. Choose menu File﹥Open Recent﹥Clear menu
3. Quit the application.

(On older systems you had to also restart the Dock (though Terminal), as a 4th step.)

As QuickTime Player is part of macOS, this requires reinstalling macOS itself.

If you describe your issue, then perhaps we can suggest ideas that are less of an effort.