Welcome to the Sound Forge user forum.
The Record transport basically works the same way as it did (in Pro 9, 10 and 11). Unless perhaps your "Spacebar and F12 Play/Pause instead of Play/Stop" setting is reversed "Options> Preferences> General". Unchecking the option box , the Spacebar should stop the transport in Playback or Record. To fully stop Record or Record-Pause, click the Stop button in the transport, which is on the bottom left in the timeline. Keyboard and/or additional toolbar transport shortcuts (Play, Stop, Pause, Record, ect., ect.,) can be added as well.
You may already be aware the 'Record' GUI is a little different from Pro 10 and prior, with the meter and other options being in the 'View> Record Options' menu.

Thanks for answering. I uncheck the option box, which you mentioned. The spacebar doesn't stop recording.

Is the 'Record Options' window open whilst recording and what are the settings? Mode= Create new Windows", ect.
Does the Spacebar work in 'Play' mode, and works normally with other apps
How about the 'Enter' or 'F12' keys?
Does the timeline on-screen transport 'Stop' or 'Pause' buttons stop or pause recording?
Maybe take a look at the keyboard driver settings and/or try another keyboard... perhaps uninstall reinstall Pro 12.1.
Sorry, I can't duplicate this behavior on my Win 10 PCs with SF Pro 10, 11 and 12.

The keyboard works normally with other apps. In Sound Forge Pro, the spacebar pauses recording only. In previous versions, the spacebar stops recording. Thank you for trying to help.

I Understand what Carlos-b9323 is saying. I have been using SF9 and the Space bar ALWAYS stops and Starts playback - BUT in SF12 - The Space bar WILL stop recording BUT It will put it in ARM mode and again - since this is for some UNKNOWN reason a feature - if you hit the space bar again - it will start recording - and it will RECORD OVER what you just did. AND:

"Spacebar and F12 Play/Pause instead of Play/Stop" setting is reversed"

I've tried it BOTH ways - when you are doing mass records you just want to hit the spacebar to stop/ start - You don't mind hitting a record button - but YOU DO NOT really want to use Function keys - ESPECIALLY since you never had to before.

Do you or anyone else know how to make the spacebar like all the previous versions or at least from SF9 on down?