I had the same problem a couple of weeks ago and had to sacrifice the stem, shame really as it was a lovely old GB stem. Managed to get the offending item out though. Just be carefully if hitting stem out from bottom of forks as if hitting expander bolt can spread the stem and even the fork steerer so you can get headset nut back on. so keep hitting expander bolt back into forks every couple of hits.
I had the same problem a couple of weeks ago and had to sacrifice the stem, shame really as it was a lovely old GB stem. Managed to get the offending item out though. Just be carefully if hitting stem out from bottom of forks as if hitting expander bolt can spread the stem and even the fork steerer so you can get headset nut back on. so keep hitting expander bolt back into forks every couple of hits.