By default ctrl+space is pause, but I'm pretty sure you can remap the buttons...
Preferences (ctr+p (or probably apple+p on mac))
Keyboard->Show actions window
Type "stop" in the text bar
Click "Space"
Click "Space" in the shortcuts section below.
Delete
Custom actions->New...
Type "pause" into the filter box
Drag play/pause over to the right box.
Name your action
OK
Type your actions name into the filter box
Select it
Click "Add" next to shortcuts (where you deleted space before)
Hit space
OK
Close out your menus, and you're good to go!