If it is because "...it is in use," log out then back in to stop the process that is in use. Then you should be able to delete it.
Some applications when (either at login or when launched) will run a background process that doesn't quit when you quit the application (e.g. iCal Helper) and as it is in use, it can't be deleted. However, because it is in the Trash, it will not re-launch the next time you log in so a restart or log out/log in will then allow you to delete it.