How to spacebar and have playhead stop at sequence out point?

Go to Preferences → Timeline → and uncheck the option “At end of playback, return to beginning when restarting playback.”

William

There's also a play video in to out button which can be added to the monitors.

Ctrl+shift+space will play in to out.

If you mean the playhead keeps on playing after the last clip then most likely you have a stray asset beyond the 'last' clip.