Hello.
As far as I know there are different PC-to-Sun adapters (those manufactured by Sun and third party devices).
These different devices seem to use different keys to synthesize the Stop key.
I found a manual of a device manufactured by "NTI" which uses Spacebar+F1 to synthesize the Stop key; Spacebar+F1+A would be Stop-A.
I do not know if the original Sun device is capable of that.
Martin