Should be. Depends on how many applications are Lock'ing the screen though.

To quit Ambient itself:
SetENV AMBIENTQUITHACK 1 SAVE
(might work first after a reboot; not sure right now)

Then you just go to Ambient's "Ambient" menu and choose "Quit" (will not pop up a requester anymore; so be careful. )

If you need to do it from a script/ shell: "Break PORT AMBIENT" should do the trick. Maybe also requires a "WaitForPort AMBIENT LOOP 10 DISAPPEAR" if you immediately want to launch your application afterwards.