Skipping the file is one of the problems I meant.
Especially if the file is still open by Audiosurf, you go to untag it and it fails. But meh, it works, I'm just pointing out potential issues for you, constructive criticism and all that.
You could remove the tags by just wiping everything after the first instance of [as- instead of needing the exact string.
Also, I seem to have broken it:
* Start YAAP, add a few songs to the playlist, tag as [as-monoonly].
* Close YAAP.
* Open YAAP, tag as [as-nostlth]
* Click Untag.
* Now, no matter what you click on in the tag centre, only the [as-nostlth] tag will remain in the text box and you're stuck with the [as-nostlth] tag until you restart YAAP.