Is there a button in iTunes to say "Stop after playing this song"?

Solution 1:

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Click on the Up Next button and press clear. It will stop after the current song regardless where you start the song from.

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"Clear" is at the end of the Up Next list, rather than the top, so you'll have to take the extra step of scrolling to the bottom.

Solution 2:

There's no command as such, but you can stop iTunes from playing after completing the current song by using the column browser to switch to any album other than the one of the current song. When the current song is done, iTunes will stop playing.

Solution 3:

No there is no command in iTunes but with Applescript you can add fuctionality. For iTunes and Applescript see Doug's Applescripts for iTunes and they have one to stop after the current track.