Suppose you've just updated to a new version of an application. Further, suppose this is one of those situations where the old version winds up in the Trash (rather than getting modified as part of the update).
In this scenario, you may have difficulty deleting the now out-of-date program. Specifically, when you select to Empty Trash, you may get a message informing you that the application cannot be deleted because it is "in use."
This is typically because the program is still open. If so, the solution couldn't be simpler. Quit the application and try again. Empty Trash should now work. With some utility software, you may have to go to a System Preferences pane to find the appropriate Quit button, but it should still do the trick.
However, with some background utilities, the situation can get more complicated. These utilities often have no obvious way to quit them. You can disable their function, but they still continue to be "in use" until you either log out (and log back in) or completely restart your Mac.
At last we arrive at the crux of the problem: You'd like to avoid the whole restart thing, at least temporarily. At the same time, you want to be able to empty the trash, including the pesky program, without getting the error message. Is it possible? Absolutely.
Some of you are probably saying to yourself: "Sure, just remove the program from the Trash until you are ready to restart." That would work. But I prefer to get rid of the old version, so that I could be certain it is no longer having any effect on my Mac. Anyway, I don't want to have to remember to move it back to the Trash later. I'm lazy that way.
Another approach might be to force quit the application process via Activity Monitor. But many users would rather not go down this "geeky" route.
There's yet another solution. It's simple, fast, and requires no third-party software: From the Finder menu, select Secure Empty Trash rather than Empty Trash. Done. While it may seem surprising, this always deletes these "in use" files for me.
The function of Secure Empty Trash is to delete files in a way that prevents them from ever being retrieved, by a file recovery utility or any sort of hacking attempt. However, I serendipitously discovered that the command also acts as a force-delete.
One caution: This may leave you in a state where the updated utility is no longer active (the old version is gone and the new version will not work until you restart your Mac). If you can live with this until you find it convenient to restart, you're good to go.