yeah, it's pretty crappy. but if you actually cared about working with Flash at all, you'd be able to rise above that.
I challenge ya, redo this on your own time, make it something you can be proud of. Put your "bullets" on a timer (setInterval) and then add a variable within your "if(Key.isDown(KEYCODE)){" that doesn't allow the gun to be fired again UNTIL that key is pressed again.
Just a variable like "var_GunFiring=1" with an extra condition in your Key.isDown "if" statement. that might look like "if(Key.isDown(Key.SPACE)&&var_GunFir ing==0){".
Please feel free to PM me or use the BBS here to get more help if you are really interested in making games.